I tell my students that if they have not flown for 2 weeks they should take 30 min and chair fly their last flight. By that I mean sit alone in a quiet area with your checklists and go through your last flight from start up to shut down, including verbalizing all the major points. so for example for take off
You would say
OK I am cleared for takeoff, throttle goes in slowly and smoothly with the stick slightly aft, good engine check done I am concentrating on maintaining centre line, rotate at XX kn climb out at YYkn keep ball in the centre etc etc.
it will seem a little bit silly at first but it can be a very effective way to retain knowledge.
The other thing worth reviewing during down times is emeregency procedures.