I’ve used this for years, and you’ll either love it or look at me funny when you hear it: anywhere that you need to frequently change numbers — say, hit points on a character sheet — just stick a piece of non-glossy clear tape over that spot, and then use a pencil to write in the number.

You can erase pencil from tape very easily, and it will hold up for years as long as you don’t accidentally poke holes in it. Enjoy!