1. Psychology.

This has to be ranked number one in my book.
If you don't possess the self discipline, patience and faith in your own system, you'll never win. Providing your system is proven and sound, you will win - but not if the right mentality isn't there.

Staking is ranked last.
Your system has to win at level stakes betting. Correct staking will only maximise profits, but if your system doesn't show a level stakes profit, it will maximise losses.