http://www.tanyakhovanova.com/MathOlymp/mathcomp.html The list of math competitions is especially useful for students in the Boston area, but there are many links to national contests.

Math camps are listed at http://www.tanyakhovanova.com/MathOlymp/mathcamps.html , and

a Math Competition Handout Archive is at http://www.tanyakhovanova.com/MathOlymp/archive.html .

It's possible that everyone here but me knows about this, but AoPS has a couple of books dedicated to learning how to succeed in math competitions. The AoPS website also has a list of other books about math contests. See this page for details.