As stated, mixture problems are the main cause of idle issues. As a first step, I always recommend looking for vacuum leaks, as this is probably the most common cause of idle problems. Here's a video showing one method of finding vacuum leaks.
Some of you might be a little nervous about using carburetor cleaner, and I understand. Know that you can also use water in a spray bottle. It might not be as effective, but at least you don't have to worry about catching something on fire. Another method is to add smoke to the intake with the engine off and look for where it escapes. They make machines to do this, and I've also seen people use a cigar to produce the smoke.
It doesn't matter what method you use, as long as it's effective at finding a vacuum leak. Once you've found the vacuum leak, repair it and recheck for your idle problem.