MathCounts

NextThought - Interactive practice problems - create a free account with parental permission

Alcumus - AMC 8 interactive practice problems - create a free account with parental permission

MathCounts Trainer - AoPS Interactive MathCounts Problems

Videos of Past Competitions

Past Competition Tests

Problems of the Week

Video Library - Great videos with demonstrations on how to approach common problems

Above are countless free resources, but if you still want more I suggest:

Competition Math for Middle School by J. Batterson