First, consider what length of hem to look for. This will depend upon the time of day and the formality of the occasion.
A daytime wedding always calls for a short dress unless the function is ultra-formal. You can wear hats and gloves if the wedding is formal.
For an informal or semi-formal evening wedding, wear a cocktail dress. For a formal wedding, look for a long cocktail dress. You can wear a gown to an ultra-formal evening wedding.
Color Me Appropriate
If you are a guest but not in the wedding party, find out what color the bridesmaids will be wearing, and choose something else. You don't want to be competing with the wedding party for attention.
However, if you are the mother of the bride/groom or will be standing up with the couple for some other reason, try to coordinate with the wedding party. This will make for wonderful photographs later. You can also ask the opposite parent (the mother of the groom if you're the mother of the bride) what she will be wearing, and coordinate with her.
Black can be worn if the ceremony will be taking place in the evening.
Never, ever wear a long white dress to someone else's wedding! It competes with the bride. Even if the bride will be wearing something other than white, it is still considered in bad taste for a guest to choose this color.
Hats are making a comeback, and it is entirely appropriate for you to wear one to a formal daytime wedding. A lady needn't remove her hat indoors; only men are required to doff headwear inside. However, you will probably want to take your hat off for the reception because it will be more comfortable for you.
If you have chosen to wear gloves, keep them short. Opera-length gloves aren't appropriate unless the wedding is extremely formal. If it is a black-tie affair, wear your long gloves only with a sleeveless or spaghetti-strap gown. Remember to take off your gloves before eating or drinking.
No matter how beautiful you may think yards of organdy and tight-fitting sleeves are, they'll be your undoing if the wedding is taking place in Hawaii. Take into account the location and time of year when selecting your dress. Velvet can be gorgeous for a winter wedding. A lovely garden party-type dress will work beautifully for a springtime ceremony.
If you're pregnant, select a flowing, comfortable but not too "baggy" dress. Today's maternity fashions can be fabulous. Take advantage of the trend toward upscale, flattering maternity wear.
If you are buying your dress well in advance of the wedding, allow for room to grow. A maternity dress that is very closely fitted in the upper waist (before the belly begins) or that has no "give" is probably not your best bet. Choose something that will grow with you, or at the very least can be let out as you get closer to the big day.
Whatever your circumstances, try to buy an outfit that you can see yourself wearing again. Fads can be fabulous, but the little black cocktail dress and the open-necked chiffon never go out of style. Look for a classic with a few lovely details and you'll be one guest who is truly dressed for the occasion.