statistics/econometrics
probability theory/stochastic processes-particularly any course which focuses on monte carlo and boot strap simulation (which means understanding calculus 1-3 and ordinary and partial differential equations)
I find game theory useful in general, and taking courses in finance/accounting can be helpful, even if just for the tax/money management side of trading.
Then again, I'm not a daytrader...but these are the courses you'd take to work at a trading desk at a bank.
