Lots of great info at http://www.batteryuniversity.com.


Amongst other things, they say that discharging to <1.0V clears memory issues, but the batteries should be exercised every 1-2 months, and definitely no less often than 4 months.


I used to charge batteries after a gig, and again before a gig (if I had advance notice of the shoot). Nowadays, with Eneloops, I charge after a gig. My non-Eneloop batteries are now "set aside" for duty in PocketWizards, where I suspect the power demands are much smaller than flash.