Memorization techniques, basically what's in _Moonwalking with Einstein_ by Josh Foer. Here's a podcast on the topic https://www.ted.com/talks/worklife_with_adam_grant_how_to_re...
Life has gotten in the way, so I haven’t been consistent with practice. Have also been procrastinated by jumping between books (Dominic O’Brien, Harry Lorayne, Nelson Dellis, others) trying to compare methods.