To this day I have to pause and mentally sort this one out in order to get it right. As with any of the other common mistakes people make when writing, it’s taking that moment to get it right that makes the difference.
“Affect” is a verb, as in “Your ability to communicate clearly will affect your income immensely.” “Effect” is a noun, as in “The effect of a parent’s low income on a child’s future is well documented.” By thinking in terms of “the effect,” you can usually sort out which is which, because you can’t stick a “the” in front of a verb. While some people do use “effect” as a verb (“a strategy to effect a settlement”), they are usually lawyers, and you should therefore ignore them if you want to write like a human.
Source : http://www.copyblogger.com