1. Decide on Formality:
In general, the later in the day the event (after 6 p.m.), the dressier. Long dresses are usually reserved only for formal (black tie) occasions.
While some fabrics are year-round -- silk, brocade, organza, jersey -- fabrics like velvet and gold spangles are best reserved for winter holidays.
2. Flatter Your Figure:
Remember that darker colors and simple cuts (a sheath, an A-line) are the most flattering.
Show off your best feature. If you have gorgeous eyes, then play them up with beautiful makeup, off-the-face hair and jeweled earrings in a complimentary shade.
Not sure what your best asset is? Ask a friend or significant other. Or go to a store that sells special occasion dresses and enlist the salesperson's help.
3. Choose a Color:
Red, bold prints, even gold are dramatic enough for evening wear. Pastels, mid-range brights and soft prints look great for dressy day occasions.
Blue (from navy to royal), pink (from cotton candy to fuchsia) and white (from candlelight to cream) look great on almost everyone. Remember that the "Don't wear white to a wedding" wisdom is still considered a fashion don't by most women.
4. Pick a Style:
Instead of going with something trendy, try to find a dress that reflects classic, timeless elegance. Remember that you'll have to look at photos of you in the dress for years to come.
Classic special occasion dresses are often deceptively simple, but they usually let the beauty of the wearer shine through. A few examples: the little black dress (a.k.a the cocktail dress), a goddess gown (draped and rouched), a ballgown, a slip dress.
Shop secondhand stores and sales to save even more money on a dress.
If you shop in a store, dress for the day by bringing along heels, any special accessories you know you want to use and a strapless bra.
When shopping online, make sure you understand both the sizing (know your measurements and check against a size chart on the site) and the return policy (some stores don't take returns on dresses.)
Although you may want to add tons of glitz, remember that you can kill a look with too much matchy-matchy accessorizing. Find earrings, necklace and bracelet that compliment each other without being a matching set.
If your legs aren't perfect enough to go bare need camouflage and you don't like to wear leg makeup, then opt for super-sheer hose or fishnets.