I just try to remember. A lot of the work I do for myself is usually impulse, anyway. If I really want to remember it, I'll at least start it, and have it sitting in the "to-do" pile to remind me it's there.
That's a good way to work it. ...actually sounds a little like what I do with writing. If I really want to do it, I'll make a note or write a little of it, and then sit it down. Although those usually get pushed aside or forgotten.