According to www.theknot.com:
1. Blowing off your budget.
Excited brides start booking vendors and making purchases without having a real budget and then are shocked to discover they’ve already spent all of their money (or parents’ money) and don’t yet have half the things they need.
2. Messing up the marriage license.
Don’t go to obtain your license the day before your wedding…you may not get it in time because some states have a three-day waiting period.
3. Ordering your wedding gown too late.
Order by the six-month mark since your dress may be custom-made and, many times, made overseas. In addition, most off-the-rack dresses will require alterations, so make certain that you have enough time to get the gown fitted properly.
4. Booking hotel rooms too late.
Don’t wait to block out hotel rooms for out-of-town guests until the last minute. If your ceremony is during a busy time and you don’t investigate hotel availability in advance, you can end up without rooms for your guests.
5. Inviting too many guests.
Make sure your guest list and your reception site capacity match up numbers-wise.
6. Last minute beauty treatments / crash diets.
Last minute beauty treatments can lead to breakouts, mistakes or, even worse, serious infections. Same goes for crash dieting in the weeks leading up to the wedding — after all those gown fittings, your dress may not fit!
7. Underpaying invitation postage.
You’d be surprised how many brides just stick a regular stamp on their invites and drop the whole batch into a mailbox. Get one invitation weighed — at the post office — before purchasing your stamps.
8. Ignoring religious restrictions.
To avoid any day-of disasters, be sure to meet with your officiant within one month of getting engaged.
9. Trying to go it alone.
Too many brides try to do it all — and this isn’t a good idea. Delegate and use all the resources that are available to you.
10. Forgetting to focus on what is important.
Some tension between the two of you (and among members of your family) is inevitable due to the sticky topics that weddings stir up, but don’t ever let things get out of control. Remember why you decided to take this leap in the first place!