Here is an excellent article from Bridal Guide about wedding dress shopping.  I agree with all the statements except maybe the one about buying early.  As always you should ask about the shipping time for the designer you would like to purchase and then add additional time for any alterations.
                 
Home > 10 Mistakes Brides Make When Dress Shopping
      
     10 Mistakes Brides Make When Dress Shopping
                                                                   Before you hit the salon, read these top bridal blunders to avoid. Browse our 
gown gallery  with hundreds of gorgeous styles.        
                                                                      By: 
                     Kristen O'Gorman Klein        
                       Home Page Abstract: 
                                      
Avoid these 10 common blunders!        
               
 Photo Courtesy of Bridal Reflections
   Bringing an Entourage
   We know it’s the most important dress you’ll ever wear, and you want to  make sure it’s absolutely perfect. But having 10 different opinions  makes it much harder to come to a consensus—and it can bring you farther  away from what you actually want. “I have always been a believer that  the most a bride should bring when dress shopping is two people,” said  Cristina DeMarco, of Bridal Reflections in New York, NY.
   Most brides do best with just their mom and maybe a sister or best  friend, said Debra Lash, of The Wedding Tree in La Crosse, WI. “They  generally have the bride’s best interest at heart, not what they want  her to wear.” One of our real bride bloggers, Theresa Misso, recently wrote about her dress shopping experiences   and said, “When you're standing there in a big dress, and random people  in the store are ohh-ing and ahh-ing and all you want to do is take it  off, you’ll need an honest, reliable companion to say, ‘I love it. But  it’s just not you.’”
   Shopping Too Early
   Many couples are opting for longer engagements these days, but don’t  start shopping more than 12 months in advance. “The moment a bride  starts trying on gowns, she’ll inevitably fall in love with one,” said  Lash. “If she’s not ready to buy, it quickly becomes difficult for her.  In other words: Do not visit bridal salons until you’re ready to make a  purchase.”
   It’s important to have the rest of your wedding details in place before  you can truly shop for a gown, since many factors can influence the  style of the dress. Consider your venue: “An ornate gown with tons of  embellishments and a sizeable train might look stunning in a ballroom  but completely out of place at a beachfront ceremony,” said Tolu  Ogbechie, one of our real bride bloggers . Also, the colors you choose for your bridesmaids and décor can influence whether you choose white or an off-white shade.
   But, at the same time, don’t procrastinate. Most dresses take anywhere  from five to eight months to come in, according to DeMarco. You can  sometimes place a rush order if you need it in under five months, but  it’ll cost you.
   Trying On Too Many Dresses
   
If you’re a fan of Say Yes to the Dress,  you’ve undoubtedly seen a bride who’s tried on 100+ dresses without  finding the right one. Most brides try on between four and seven gowns,  said DeMarco, and shouldn’t try on more than 10, because it can lead to  confusion.
   Often, a bride is trying on dress after dress because she’s just not  ready to end the shopping experience and make a real commitment. “The  most common mistake that a bride makes is not purchasing her gown when  she finds it,” said Lash. “The danger that befalls her is she becomes  overwhelmed and forgets what she originally set out to find. She begins  to doubt herself and starts to lose faith in the experience.”
   Not every bride has “that moment” when the tears start rolling and you know you’ve absolutely found “The One.” Real bride blogger Crista kept searching until the tears started rolling . While trying on a Priscilla of Boston  gown (pictured right),  Crista noted, "As I gazed upon my reflection, I noticed my family  standing around me, teary-eyed. Although I was touched by their  reaction, I wondered, ‘Why aren’t I crying?’” said Crista. “I decided  not to purchase that dress and left the salon contemplating my shopping  experience.”
   Being Swayed by Steep Discounts
   Try not to be too tempted by the lure of your dream designer gown for  70% off. “The biggest mistake I have seen brides make is purchasing a  gown online or from a shop for a deep discount ‘only today,’” said Lash.  “You get what you pay for.”
   Sample sales are popular among brides, but you must keep in mind that  you are typically truly purchasing just that — a sample: The dress has  been tried on by hundreds of brides and could have been torn, stained,  or otherwise damaged during the process. Before making a purchase, check  the gown very carefully for any damage—and if there's anything that  can’t easily be fixed, put it back. Some brides end up spending hundreds  in alterations to fix what’s wrong with the sample when they could’ve  found a similar brand new dress for not much more money.
   
But  don’t be totally discouraged: Sometimes you truly can find an amazing  gown at a bargain price. Kimberly Sanicki, of Wanaque, NJ, found her  perfect wedding gown (left) at a “Running of the Brides” event thrown by Filene’s Basement. She found a stunning Essense of Australia gown for just $500, marked down from $3,000 .  Before handing over the cash, Kimberly and her mom thoroughly examined  the gown to check for any flaws. “My mom made sure it had every crystal,  and it did not need to be cleaned or even hemmed,” said Sanicki.
   If ordering online, check the return policy carefully. If it says the  dress is sold strictly as is and can’t be returned if damaged, close  that browser and back away from the computer. One of our real bride bloggers, Tolu, successfully purchased her gown online  at a nice discount. She warns other brides, “There are a lot of websites that offer dresses made overseas .  This could be a cost savings, but I do not recommend it. There is a  very small chance that the dress you receive will actually be the one  shown in the picture.”
   Demanding the Wrong Size
   Wedding gown sizes typically run one or two sizes smaller than street  clothes, so try not to get hung up on the number. “Do not fixate on the  actual size but instead on how your measurements compare to the  particular line on the specific designer’s size chart,” said Gesinee of  Gesinee’s Bridal in Concord, CA.
   Planning on losing weight before the big day? “If I had a nickel for  every person I have measured who said they were going to lose weight, I  would be rich!” said Lash. Play it safe and order a size that fits your  current body. Any dress can be taken in, typically up to four sizes  without affecting the look of the dress, but most can only be let out  one full size—if that. “You need to be realistic, in case you don’t lose  your targeted weight,” says DeMarco.
   What happens if your dress doesn’t fit when it comes in? First, don’t  freak out and go on a crash diet. “There’s always something you can do,”  said DeMarco. “You can order fabric from the designer, add beading, or  even put in a new back to accommodate the measurements needed.”
   Photo Credit: Timecut Studios (left); Courtesy of Priscilla of Boston (right)
   Next: 5 more common gown-shopping mistakes >> 
   Designer Henry Roth checked in with real brides to see their  biggest fears and stresses when it comes to buying a wedding gown —  plus, great advice from brides who have already made their purchase!
   
    
 Photo Courtesy of The Wedding Tree
   Sticking With One Style
   “Ninety percent of our brides leave with something completely different  than what they had in mind when they walked in,” said DeMarco. Bringing  in pictures of dresses you like is a good start, but if your consultant  recommends something different, give it a try. It’s impossible to tell  what a dress will look like on your body just from seeing it on a  hanger. “If a bride is fuller in the hip area, she might have a mind set  that she can’t wear a fitted dress,” says DeMarco. “However, what she  may not know is that a more fitted dress would show off her curves, fit  beautifully, and make her appear more slender as opposed to covering  up.”
   Need advice on which shape is most flattering? Get some recommendations here .
   Buying a Dress You Don’t Love
   “The biggest mistake I’ve seen brides make is settling on a dress  because someone influenced them to purchase it instead of going with her  heart,” said DeMarco.
   This is why it’s so important to choose your shopping companions  wisely. “Many people that may accompany the bride will look at each gown  and decide if it’s best for them—not for the bride whom they should be  helping,” said Lash.
   If you find yourself liking a dress but also having doubts, or if  you're feeling pressured from a part of your entourage or a salesperson  to make a purchase, it's okay to go home empty-handed, sleep on it, and  return at a later point if you're still dreaming about the dress.
   Trying On Dresses You Can’t Afford
   You see a stunning gown  a mannequin that’s way out of your price  range, but you want to try it on just for fun. What’s the harm? “It can  lead to heartbreak and disappointment,” said Lash. “You need to be able  to afford anything you try on.”
   Make sure you are completely clear on your budget before you hit the  salon—and make sure that the salon you choose respects your budget. One  of the top complaints Lash hears from her brides is that other salons  showed them dresses that were out of their respective budgets. “We have a  strict policy in place to keep the customer within budget, which makes  her happier with the entire process,” said Lash. “Happy bride equals  happy life for bridal shop employees!”
   Not Accounting for Extra Costs
   Let’s say you have $2,000 set aside for your attire. That doesn’t mean  you can buy a dress with a list price of $2,000. Alterations typically  cost extra, and you may also be charged tax and delivery, so don’t max  out your fashion budget on just the dress. “Almost every gown will need  some alterations,” said Gesinee. Whether it’s something, small like a  hem, or something major, like taking in (or letting out) the bodice, it  can add up. You'll also need to account for undergarments, shoes, a veil  and/or hair accessories, jewelry, and any other accessories you'll want  to complete your bridal look.
   Not Choosing Your Store Wisely
   If you’re a plus-size bride, call ahead to see if your desired store  carries a sizeable amount of plus-size samples—not all do. Otherwise,  your only option is to play “paper dolls” to get an idea of what a style  would look like on you, said Gesinee.
   “It is very hard to determine fit by holding up a gown to your body,”  said Lash. “If a bride loves one of our gowns in a size 12, we will find  one with an exact or similar fit in her size to try on.”
   Also, make sure that your store is an authorized retailer for the  designer you’ve selected, said Gesinee. This is especially problematic  when buying online. If the retailer isn’t authorized to sell that  particular designer, there’s a good chance that you won’t receive an  authentic gown. That means that the designer won’t stand behind it if  anything is wrong. To find out which stores are authorized in your area,  check the individual designers’ websites or call the number listed.