I'm ten years out, and had some of the same worries. Along with my request for a letter, I sent my old professor an old paper I'd written for him, my resume, a picture of me at the time he knew me, etc. The picture jogged his memory, and I got a promise of a very enthusiastic letter from him, whie the other stuff I sent him gave him what he might need to make it a better letter in case he didn't really remember me all that well, which I'm certain was the case.