The distinction between “affect” and “effect” trips up lots of people, but with one or two little mnemonics, you can master this tricky pair in probably 95% of cases.
Effect is almost always used as a noun meaning “the result of some action.”
Affect is almost always used as a verb meaning “to influence or bring about change.”
Affect, which is an action (another word for “verb”), starts with an a, like action. There’s your first mnemonic. When something affects something else, it has an effect. The affect or verb happens first and the effect or noun second, just as affect comes first in the alphabet and effect second. There’s your second mnemonic.
My experience with mnemonics tends to be that once I’ve had to use them enough, I internalize the underlying grammar that they help me to remember so that I no longer have to remember the little hint. So the affect/effect distinction comes naturally to me now without the memory tricks, and maybe it will for you too one day, if this is one of those distinctions you struggle with.
This entry wouldn’t be complete if I didn’t complicate things a little. There are of course alternate uses for both words. You can speak of a person’s mood or way of outwardly presenting his mood as his affect (in psychology you’ll hear of a person’s “flat affect”). And you do sometimes hear of someone “effecting change.” These other uses, along with the fact that the two words look and sound similar, are what wind up causing all the confusion. But these are pretty specialized and uncommon uses, and if you remember that affect is almost always an action or verb and effect almost always a noun, you’ll almost always be correct in your usage.