I do two things. First, I've begun writing my versions (scripts and mechanics, separated by stage, parlor, close up, impromptu, etc) into word documents and organizing them on my computer. I've only recently started this, because until now I didn't have to worry about remembering everything.
The second thing I do is simply not to worry if something gets rusty. Everything I use is up to snuff, the things I don't use I don't worry about. I keep the reference material around so that I can refer back when creating new routines or working on new material, but until that time I just remember that the material exists and where it is. If I ever need it, I can think to myself, "Hm. I need a card control that takes the card from here to here ... I think the Suchandsuch can do that. I'll have to look it up." Whenever I'm performing, I already know everything I'm going to do so I don't have to try to scrounge for new methods.
I guess I actually do three things. I also try to learn at least three methods to any trick I do regularly. That way I can use the various methods to cancel each other out when I have to perform the same trick to the same people, due to lack of new material. I've also learned that presenting the same trick in a different way will help. People already saw your coin bend? Do a bottle-cap bend instead. Same method, different thing being bent, so it seems totally different.