Color grading is an often overlooked step in the video production process. It is one really big thing that will take your projects from looking amateur to looking professional. Grading, or "color-correction" as it is also called, has a two-fold purpose. First, it is to improve the overall color and help each shot to "pop." Second, it is to create a unique look for your production when you want something different. Grading, the final step in the production process, should not be an afterthought, however. The best results come when you shoot your footage with the intention of grading, preserving the maximum amount of detail and information in your shots.
A couple of guys have a series of YouTube training videos called "Shoot Your Friends," and one episode is a quick overview of grading. The production value of their videos is not great (audio especially), but the content might be helpful. For more before/after examples of grading, take a look at the demo reel from colorist Gerry Curtis.
While grading is best handled by someone who specializes in that particular art, anyone can see improvement in the look of their videos by learning how to use programs they already have, such as Apple Color or even built-in tools available in editors such as After Effects, Final Cut, Premiere Pro, etc. The biggest thing is to leave yourself time (and money, if outsourcing) at the end of your production schedule to do proper grading. On several occasions, I have had to send projects out the door without a final color pass simply because I was up against a deadline. Planning ahead will give your productions the best chance at success.