You may have read it already, but it's working linking to over and over again, Jeff Atwood has an excellent list of warning signs for Code Smells. Be aware of these warning signs. If you are seeing them in your code, then...I'm sorry. I really didn't want you to have to find out like this. Hehe.
A truly excellent list and something every developer should read. I've been guilty of some of those from time to time myself, but live and learn, right? As long as you are aware of those things that make your code downright stinky, then you can at least learn to improve.