I would have really liked to call this blog post "How to win a Photo Contest", but in truth, I've only won once (DWR.com Polaroid Contest, see below) and placed Honorable Mention (APA) another time, and both of those were years ago.
Right now I'm entering as many legit contests as are appropriate to my work, meaning I'm not entering wedding contests or sports contests or best sunset contests. And here's why; I certainly wouldn't turn my nose up at any prize money, but what I'm after is the publicity. More specifically, I want the free publicity and the website traffic a win will come with, not to mention my own efforts on twitter, fb, etc.
Ok back to the point, the first thing you need to know about entering contest is who is the judge (or are judges). This is crucially important. Usually it will be listed, if not you probably don't want to enter that contest anyway. Without this info, chances are you're wasting your time (unless you got a shot of the Lock Ness monster).
Next you want to do your research on the judges, who are they, what's their background, where do they work, what do they do. Based on this information, you need to put together a series of your best images that you think those judges will respond well to. If you're entering a serious contest, pictures of babies, puppies and flowers aren't going to cut the mustard (unless your name is Robert Mapplethorpe).
Now with all that info, you're ready to upload, right? Wrong. Chances are you will need to write something about your work, so get that done in advance. You're going to need to write it in that fancy talk you see on the programs at the museum. Yes, seriously, judges looks for that because its important to them that not only do you know how to take a picture, but that you also have a way to verbalize your concept. You don't actually have to do this part yourself, ask a friend to write it for you if its not your forte.
Ok, now you need to figure out how many images you want to enter. Sounds simple but its not. Lets suppose you are allowed up to 15 images, how many do you actually need to tell your story? For instance, for the Artists Wanted competition, I only entered 6 images although I have 21 in the series already. Would adding 9 extra images have plead my cast any stronger? Or would I have bored the judges after the first dozen?
Remember that the judges are professionals, and will be very sensitive to filler. Don't give them more to look at than you need to win. Given the option of showing 6 of my best images and all of them, I chose 6 of the strongest, and reduced the chances that the judges would start comparing my images to each other, winnowing out the less strong or just getting bored.
So that's it; find the judges, do your research, write something good, edit your entry to the minimum you think it will take to impress the judges. Good luck, and good shooting.