@PearsonArtPhoto has a good list started. I've been getting into contesting lately and I would elaborate on No. 7 to say that for good scores, you need to plan your operating time carefully and then have your butt in the chair the whole time you plan to operate.
Some contests run for, say 48 hours, but you can only operate 36 hours, or something like that, it varies. This is where planning comes in:
- How much operating time can I get away with and stay married? (YMMV)
- How do I break up my operating time to maximize my score, take advantages of openings, etc.
- Given those answers, arrange sleep and food so when I am at the radio, I am fresh and not distracted.
- If you don't talk for a living, consider your voice. I don't and I didn't and I went hoarse my first few contests. My wife came up with a nice tea that stretches my voice, but I decided a voice keyer was needed to extend my time.
Continuing on that list:
You learn the limitations of your equipment. With experience you can tell which pileups you can just skip and how loud (S meter wise) a signal has to be for you to have a good chance of getting him to come back to you.
If you do any running, you will learn to pull call signs out of a pileup and that helps with copying information in adverse conditions.
Also expanding on both the previous answers, you will learn your radio better. I always end up pushing a button I don't mean to in the heat of battle and then have to diagnose things on the fly. And to tease out those weak signals without (in my case) any fancy DSP, you learn some more of those knobs to eliminate adjacent signals as much as possible.
Lot s of folks hate contesting and say those QSOs with their ritual "59" RSTs don't count for much. In some ways I agree, but it's just a different facet of the hobby. Instead of a quality contact, it's about quantity and, as we've tried to show here, uses some different skills or the same skills in different ways.