These are some generally useful documents.

IMC.pdf
    An IMC training document by Mike Daas, with good review of many areas,
    and many practice problems. Copied from:
    https://pub.math.leidenuniv.nl/~daasma/IMC.pdf

polya.pdf
    A summary of Polya's problem solving techniques, from his book
    "How to Solve It". This is not specifically for math competitions.
    It is good general advice, but less relevant to math competitions.