We have two types of gig; the pub gig - where anything goes - its less well paid, therefore we'll play what we like and the function gig where we are well paid and provide a service centred around dance music.
We have a get together away from rehearsal to decide on new material, if its on the list it, it gets learned and rehearsed in list order - we'll change the key to suit the vocals if necessary and record a version of the song and listen back to decide if its good enough to play live.
Two 'lessons' that I've learned - its not what you play, its how you play it and play to the strengths of your band.