Plan ahead

When you're off to pick up a large bookcase someone has hired you to paint, and you have the choice between the 10-foot moving truck or the cargo van, take the truck. Or you'll have to take the bottom cabinet back to the workshop, go back to the rental place and trade the cargo van for the pick-up that (luckily) was just returned, then drive all the way back to the client's house to get the larger piece.

Conversely, being friendly to the rental man may result in him just trading vehicles so you don't have two rental charges, and then when the computer refuses to charge for the second leg's mileage, he'll just say "it's your lucky day" and send you on your way.