Always consider the occasion. How formal is the gathering you are attending? If you are attending an upscale dinner party, you should stick with a more conventional cut and avoid a dress that falls above the knees. For more informal parties, fashionable cuts that show a bit more skin are perfectly acceptable but should remain tasteful. A cocktail party dress in a conservative style works well for both.
Find the perfect color. Consider what colors accent your best features. There are many online sites that give color recommendations based on skin tone, hair and eye coloring. You can also ask your friends and family which color you look best in. For example, if you have brown or olive-colored skin with darker hair and eyes, jewel-toned dresses will look great on you. Lean towards deep, rich purples, pinks and golds for your party dress. If you have ivory or beige-colored skin, choose a formal party dress that brings out the undertones in your skin. A royal blue or teal shade can accent skin with bluish undertones while skin with pink and red undertones looks lovely with peach colors.
Always consider the fabric. While a dress might be cheap and in a cut and color that you like, a low-quality fabric is always noticeable. Spend the extra cash to buy quality materials, and your party dress will look nicer on your body and to the discerning eye. Also, pick fabric that will minimize any problem areas and accentuate your curves. Jersey and knit fabrics can cling to your body and make you look bigger than you are, depending on the style. These can also make the dress look more casual and are not appropriate for formal party dresses.
Don't worry about following trends. The key to a good cocktail party dress that will last you through multiple wears is to choose a style, cut and color that complement your body, features and skin tone. Empire waists and A-line styles are flattering on most body shapes, and ball-gown cuts are stylish as well.