To avoid losing stuff or leaving it behind:
1. Develop a system where individual gear items and cases are always packed in the same way into the vehicle(s). This way you'll notice if something is missing, both on the way out and on the way back. Have one person responsible for placing the gear into the vehicle while the others (not including the singer, obviously) bring the stuff out of the venue.
2. When leaving the venue, always have a (sensible and sober) band member walk slowly around the stage area checking to see if you've left anything behind (leads, monitors, drummers etc).
3. And the last check before you drive off - walk all the way around the packed vehicle, looking for anything left on the pavement, propped up against a wall or tree etc etc. Stuff is easy to miss in the rush to get away. At different times I've found one of our monitors and a guitar left outside.