AoPs- Best Practice Site! Make sure you explore all of it!
http://artofproblemsolving.com/
MathFights- Challenge others!
Preparation Drills for MATHCOUNTS
MathCounts Minis- Explanations for How to Solve Certain Problems
http://mathcounts.org/resources/video-library/mathcounts-minis
Things You Need Memorized to Succeed in Competition!