This may not work for everyone, and it's a bit expensive up front, but in the end it's no more than you would have spent over time:

when I started LC-ing, I went to the Levi's store and bought a pair of Levi's jeans in sizes 18, 16, 14, 12, 10, and 8. (Yes, I got a weird look from the saleswoman in her size 2's.) I can fit into the 18 and the 16 now, and the others sit there on the shelf and smile at me. Next time I need to buy new clothes, I can try on the jeans I already have to determine my appropriate size. I always feel more confident in a store if I know my size. It certainly cuts down on the "Do you have this in a bigger size?" nightmare. And of course, I would have bought the jeans anyway, so it's nice and convenient.

I would only recommend doing this if you have a certain style of pants (like jeans) that you really like and won't want not to have. It's also a great motivation. Just resist the temptation to try on the 8's--there will be tears.

Ciao,
Kypraia