Problem solving strategies by A. Engel:
and Problem Solving through problems by L.C. Larson are both great:
https://math.la.asu.edu/~ifulman/spring13/mat194/problem-solving.pdf
Putnam and Beyond is also a great source of good problems: https://www.amazon.com/Putnam-Beyond-Razvan-Gelca/dp/0387257659
Tbh, I can't tell you from personal experience because after I got into university I stopped preparing for competitions and use my free time to study some other math instead.
Anyway, here are some books that I've heard are good for Putnam-esque preparation.