In statistics, a type I error is when you reject the null hypothesis when you should not; a type II error is when you accept the null hypothesis when you should not**. No owner of a normal brain can remember which is which without problems. Ethan Fosse posts a mnemonic which is supposed to help, but it doesn't work for me. Neither do any of the alternatives suggested in the comments. So I came up with my own. Here it comes.
The null hypothesis is often represented as H0.* Although mathematicians may disagree, where I live 0 is an even number, as evidenced by the fact that it is both preceded and followed by an odd number. Even numbers go together well. An even number and an odd number do not go together well. Hence the null hypothesis (even) is rejected by the type I error (odd), but accepted by the type II error (even).**
*Also, "null" means "zero" in German, but you don't really need that.
**Added: O.k., o.k., you don't actually accept the null, you just fail to reject it.