I was killing time online today, and no less than five people I chatted with mixed up the word “affect” with “effect,” or vice versa. Even some grammatically competent people I know have trouble with this one, so here are a few tips you can use, even if it's only to humiliate your friends. Both words have noun and verb forms, so that's not always a dead giveaway. But most of the time you hear them, “affect” will be used as a verb, while “effect” will be used as a noun.
Affect: as a verb, to affect something is to change it. Here's an example: “Britney's VMA performance affected my ability to watch the rest of the show.” As a noun, it's usually used in psychology.
Effect: in it's less common use as a verb, it means “to make [something] happen”. For example, “Americans were able to effect a change in tax laws.” As a noun, an effect is a result.
Remember: When you affect something, you have an effect on it.