I clean used bands with an ultrasonic machine, scalding water, and an industrial grade de-greaser. I stretch the bands over a frame to open up the crevices. This gets most, but not all of the gunk out, because for most bands, it is impossible to get it all out from under the caps. As noted, for the twist-o-flex bands it is easy to remove or replace links, and I keep a box of pieces. For the scissor link type bands, some came with removable links, but most did not. There are appears to be a de-factor industry standard of short-regular-long that applies to bands so these were probably supplied that way. I have a few bands that are too small even for my fairly small wrist so I'm guessing they were the short variety.
I have repaired scissors link bands by removing the caps and replacing springs. I have also replaced links but this requires some tricks with the flared rivets and is a real PITA, so I usually just look for another band.
I sometimes buy a watch just for the band. For instance, I once got a GP Kestenmade mesh band at a flea market on a $5 watch. The band was worth 10 to 20 times that.