Old School House Rule #1 Unlimited Spells Known

Some older editions and retro-clones limit the number of spells a magic user or other class can put in their spell book.

To my way of thinking this is unnecessary and does nothing to actually balance the class. The real balances on a MU's powers are what spells I as DM allow them, slots per day, and what magic items they some arbitrary limit.

So I simply drop that rule.

Another way to do this if you want less sharing spells or looting books is to keep the limits but exempt spells the magic user personally researches from the limit. This has the salutary effect of increasing research efforts and customization which can be fun.