One of the common pieces of advice found in test-prep books is to avoid going to the exam dressed in your pajamas. The logic is that it's good to be comfortable, but you also want to feel competent - and who feels that way ratty pair of sweatpants? My go-to outfit for travel and test days involves a long cable knit sweater and ankle length black leggings, probably not exactly the what the Princeton Review professionals have in mind but it looks nice and also allows me to sit cross-legged in the chair.
When it comes to big tests like the MCAT, the LSAT or GRE (which I took last Thursday) I have a different little confidence trick. After I've done all the studying that I can possibly do, I get my nails done. Midway through the test when my confidence gets rocked, there's something about having put-together-lady hands that makes me feel like I've got this.