@wildflower: I guess I know what you mean, but I’m generally pretty good at imagining what would look good on me. Although I do have a small stack of stuff in the back of my drawer that I ordered and then realised looked awful :). My main reason for ordering is that I quite like the odd printed shirt—although most of my stuff is MUJI style—and the problem with print shirts is that you often see people walking around wearing the same stuff as you if you bought it locally, which I don’t like.
@Allie: It’s really only Threadless and American Apparel that immediately spring to mind, but I can occasionally find something cool on crappy websites like Truffle Shuffle.