I will probably add to and tweak this list for some time:
1) Avoid thirds in the bass. They make your teeth clack together because of the tempering. Only stick them in if you’re writing something to be played on strings.
2) Thirds in the treble are better. They’re still tempered, but the beating isn’t as noticeable the higher you go.
3) Use changes in texture; don’t make the music too isotropic. Don’t make the whole thing a blizzard of 16th notes, or keep your hands over the same rough area of the keyboard the whole time. You’ll bore people to tears. A given piece of music needs to evolve as it goes.
4) V-I is probably the simplest, shortest piece of music ever written. It’s like that shortest poem ever written, in trochaic monometer, called “Fleas” by Strickland Gillilan:
V-I is like that, and can be thought of as going from a half-step beneath the tonic to the tonic, like B –> C. It’s like subject-verb in a spoken sentence.
5) Write a piece of music like you’re writing an essay. Make a thesis statement with a few sub-parts, maybe three different gimmicks. Expand on each gimmick, and then go back and restate the thesis in a new way, with insight on each gimmick that you gained from having noodled with each.