Retuning

If you can, leave your bass on the stage before you start playing, that should reduce the temperature change.

if you have a tuner it's easy, just mute the output and tune whenever the frontman talks if you hear it's going out of tune.

Depends on how great the tuners are. My 350 dollar SG has some of the best tuners for a cheaper guitar, when my drummer and I practice the only time I really retune is to drop the tuning. I usually like to start out in E, then flat, then D, then whatever other tuning of songs we know. That's the best way to do it so if you don't have a backup and don't want to constantly tune just go to standard then work your way down.

My jam sessions are epic, sometimes up to four hours at a time. I'll load up a play list of songs to jam to, listed by tuning (I start with songs in standard, than work my way down to D-tuning, sometimes Drop C), and it's not uncommon for me to play through forty or fifty songs in a single sitting (and sometimes more!). So I do find my tuning going sour every now and then and having to re-adjust. I'd say, oh, every six or seven songs or so. That may seem too frequent to a lot of folks but then not a lot of folks give their basses as hard a work out as I do LOL