School math is routine and boring, but what kind of mathlete are you if you cannot do it? Register as a tutor on http://www.algebra.com and make sure that you can solve any of the problems posted there by your math-challenged peers.
Art of Problem Solving
http://www.artofproblemsolving.com offers a set of textbooks based on math competitions that covers middle and high school levels. A lot of problem sheets are available for free download. They also keep an active online forum.