Knee pads such as those used for in-line skating or volleyball are a really good idea. Safety glasses or sports glasses prevent your eyes from getting watery from the wind if you skate really fast. Elbow guards are another option, but if your budget is tight, these could be the first to go. Pants should allow for lots of flexibility at the knee and waist, yet not be so loose that they flap in the wind. Jeans are a bad idea because they're constricting and they chafe.
Sweat pants are good, and lycra running tights are the choice of the fast and the fashion conscious.
In the summer, bicycling shorts or lycra shorts without the 'shammy' in the crotch are very fashionable. On the top, try not to pick something that is so loose-fitting that it's wind resistance slows you down. A T-shirt or a cycling jersey is perfect. If you wear a wind breaker, make it a close-fitting one. Try one a few sizes too small or one designed for cycling.