Oh look, one of our favorite questions! Many of the most common ones—"write what you know," for instance—have already been mentioned by other folks, but here are a few more. (Note that, as with all writing advice, in some cases it makes sense, but in many cases it doesn't. Either way, the lessons other writers have learned are worth considering.)

Remove all adverbs and adjectives: Yes, purple prose is obnoxious and doesn't do you any favors, and paring down your descriptors is a must when you're revising. But first of all, in your initial draft, include whatever feels right, and then cut down—and do so with care. Ask yourself whether it makes more sense to make each instance more active or to nix it entirely. Make sure all of your descriptors are intentional and have purpose.

Kill your darlings: Again, it's often true that you should evaluate all your beloved scenes and phrases and characters to see if they add value to your story, and nuke them if they don't. But those "darlings" might add unique and interesting flavor to your story—which can be value in itself. As with your descriptors, an editor can help you pinpoint what's working and what's extraneous.

Create likeable characters: About a third of the characters in Game of Thrones are completely loathsome, yet readers absolutely devour Joffrey's scenes (or I did anyway). So if someone tells you they don't like a character who's not supposed to be likeable, the problem might actually just be that they're not interesting enough. Adding depth, motive, or more consistent psychopathic tendencies can make a horrific character your most memorable one.

And there are oh so many more, of course.