I’ve played around with mechanisms for spell retention many times since I first began playing AD&D a long time ago; my first attempts were very clumsy, and later I developed a system of spell points which was ridiculously complicated and required me to rewrite and modify almost every spell. Needless to say, they were all tossed out eventually.
This system is, to me, far more elegant and also a bit more chaotic, which I think fits the Old School feel much better. It is a deceptively risky system, utilized rarely by my players since they’ve gotten to know it more closely; the minor damage suffered can be devastating for a lower-level wizard, and at higher level the risks increase drastically.
The fact that the entire rule can fit on one page, and that I’ve playtested it with good results for a while, means I wanted to share it here.