i just show up, click on people, and complain about how bad everyone on my team is
(i play medic)
the more you practice the less your aim will suffer between gaps of not playing
(if practicing aim in a videogame is anything like practicing an instrument)
in the end (consistent, productive) practice can only significantly reduce how much your aim suffers when you come back to play
15 minutes of quake will help you adjust quickly
also tr_newbots which hasn't been mentioned yet