We recommend the following articles, books, websites, and contests.
The Art and Craft of Problem Solving by Paul Zeitz
The Art of Problem Solving series
Countdown by Steve Olson
Geometry Revisited by H. S. M. Coxeter and Samuel L. Greitzer
Mathematical Circles (Russian Experience) by Dmitri Fomin, Sergey Genkin, Ilia V. Itenberg