Check out Positive, Comparative, Superlative by rogersgeorge. Here is an excerpt:
Been a while since I mentioned this fairly common error (but I have mentioned it. More than once.).
When you compare two things, use the form that ends in -er: steeper, sweeter, longer.
When you compare three or more, the winner gets the -est form: steepest, sweetest, longest.
The full article is available here.