We put our NiMH packs away after the race with whatever charge is left in them (usu. 1/3 to 1/2 charge remaining on 4200 or 4600 packs) for up to two weeks. If they're going to be stored longer than that I prefer to charge them up to 80 % or so, then cycle them (discharge/charge) on the charger every 2months or so.
I discharge all matched sidexside packs to 0.9 V per cell and balance them in a tray before recharging the night before race day. Yes it takes a lot of time, but its well worth it for the consistent performance. If all cells don't discharge equally they won't peak at the same time either.
All our packs are repeaked immediately before the race for stock or 19T racing and it makes a HUGE difference in initial power output. Mod motors don't seem to be affected the same way so I don't always repeak unless its for a really long main. During the day I let the packs cool but don't discharge before recharging them. Usually they're even better on the second charge! By the third same-day cycle they seem to go flat - good run time but loss of punch - so we limit packs to 2 uses each race day.
Stick or unmatched packs I discharge with a Spintec Battery manager that somehow does an amazing job of keeping them balanced, then charge and store the same way.
Here's the advice from battery experts:
http://promatchracing.com/ibins.php