I started to buy a few new clothes after I lost about 15 lbs or so. I actually bought a few things that were a little tight to start with. I'm working on losing a total of 80lbs or so, and I have lost about 50 so far. I work full time as a physician, so it just wouldn't have worked to continue to wear clothing that was too loose. I have been skipping sizes. I started at about size 20 or so, and wore a size extra large lab coat (which was too tight). After losing 30lbs, I was able to fit into a size medium lab coat. The first new clothes I bought were size 14.
I actually wore my size 14 clothes a little too long, and I skipped size 12 entirely. Earlier this week, I tried to wear a pair of size 14 pants, and realized that they were so loose that I was in danger of losing them. They were without belt loops. Luckily, I gave them a tug before leaving the house--they came off entirely without unzipping. So, all the size 14's are bagged for Goodwill. I haven't lost as much on top, so, my large tops are still wearable for the time being.
Buying the smaller clothes a little tight gives me an incentive to continue losing weight and encouragement when I notice that what I once could barely zip is now loose.