Urgh! I'm terrible with that one too. Especially in science stuff. Sometimes I think the people who correct it, don't really know either, so they make it worse.

When you affect a situation, you have an effect on it is what I tend to remember.

According to the site there are actually five different meanings for affect/effect. (http://www.wsu.edu/~brians/errors/affect.html) No wonder it's easily confused. lol How awesome is that site though??