My advice when looking through catalogues whether online or in-store (this is how I actually buy for the store). I cover the model's face and then the dress stands out more objectively. Sometimes the model is so beautiful that she can make a potato sack look couture.
Test this out for yourself, you'll be astonished at the difference you notice in the dress.
Mother of the girls, mother of the groom, or wether you are on the lookout for your dream cheap prom dresses online, it is a great idea to look through online catalogues jovani, mon cheri, cm couture, etc. to see what you gravitate toward. Having said that, always keep an open mind when walking into the store, because what you love might not be the best silouette or colour for you.