Keeping a list of favorite things (not necessarily problems) that you keep coming back to. This helps linking ideas and finding connections (I guess?).
References
Quote
Richard Feynman was fond of giving the following advice on how to be a genius. You have to keep a dozen of your favorite problems constantly present in your mind, although by and large they will lay in a dormant state. Every time you hear or read a new trick or a new result, test it against each of your twelve problems to see whether it helps. Every once in a while there will be a hit, and people will say: ‘How did he do it? He must be a genius!’
Gian-Carlo Rota