I have a simple one that I use often. In fact, it's attached to my computer with a Post-It note. It's a simple affirmation that I firmly strive to keep close to my heart:

           TODAY I GIVE MYSELF PERMISSION TO WRITE CRAP.

I think a fundamental trap when it comes to writing is the belief that you output that day must be a perfect piece of prose. This can freeze up a writer. I've seen it happen before, and I've felt inklings of that pressure myself. That note is a reminder to shake that off and do one simple thing that day: to put words on a page.

Well, let's talk about what else is attached by Post-It notes to my computer. One is a single word: SPECIFICITY. It's a reminder to make sure that I don't go for the general word (like tree) but go for the specific (like oak or elm). There is power in "specificity" that makes prose shine more strongly, but it's easy to slip up and go for the easy generalization. That writing prompt is there as a daily reminder. Likewise, the other Post-It note lists the five senses. Again this prompt it to remind me not to describe that movie playing in my head through only visual descriptions, but to employ the full power of other senses to make that scene come fully alive for the reader.

This came from an old pulp writer from the 30s and 40s. His name was Lester Dent, but he wrote under the penname Kenneth Robeson. He penned most of the Doc Savage novels, books I read avidly as a kid (okay, just to clarify, I read the Bantam reprints of these old pulp novels during the 70s). But he left behind a great nugget of advice: "Never kill your characters the same way twice."  It's a simple bit of wisdom, fraught with layers of meaning in regards to the pitfalls of repetition, the dangers of lazy writing, and the need to keep both you and your readers on your toes. I've drilled that bit of knowledge down so deep that I don't even need a Post-It note to live by Lester Dent's philosophy. And all writers should.