100% let them know about a cash tip, be generous too. They're handling all your stuff, if you want them to treat it right they've got to be incentivized to do so. It's a hard job. If it was easy money everyone would be doing it.

When you do tip, give each guy their share directly.

Also buy a bunch of gatorade and water or other drinks and put it in a cooler with ice and tell them to help themselves to it, especially if it's at all warm where you are.

Finally, don't be a hard ass and don't let them walk over you either. Don't get on their case about stuff that actually doesn't matter, and be firm where it does matter. You're paying them to do a job, but they're more likely to respect you and your stuff more if you don't treat them like crap (like some people undoubtedly do - they've told me).