Help! I'm wasting so much time with returns/exchanges!

I'm new to buying women's clothes for myself and I'm only doing it online because I haven't built up the courage yet to walk up to a sales person and say "Hi, could you help me find a skirt that's my size?" So far, I've bought a couple blouses and been successful on the first try. And I lucked out with a great little black leather mini skirt...ooh, ah. But, I've had horrible luck at other stores getting my size right. For the record, I'm 6'3", weight 195lbs. and am 43-36-39. But here's an example of the problems I've run into: I ordered a couple similar skirts from GAP. According to their size chart, a 351/2 waist is a size 18 and a 371/2 waist is a size 20. Their sizing instructions said to leave a finger between you and the tape. So I ordered the 20. Well, the skirts were too big. So I called customer service and they said I should have sized down to an 18. Well, the 18's arrived and they were too big. So I exchanged them for 16's and THEY are too big! Hey, I wouldn't mind having a cute little figure, but this is getting ridiculous! What's a girl to do? Is it this particular store/manufacturer? Am I doing something wrong here girls? HELP!

Besides, fractlgrrl is flaunting a beautiful silk georgette dress from Chadwick's in her profile pic and if I can't have it, in the right size, I'm going to scream! :drool:

Carson