one extremely easy solution comes to mind since it is exactly what my tarpmates and I do every night.
When it is time to take up your tarp, pick up every single thing that is there... cups, plates and all that... and TAKE it with you to deposit in the proper green bin. We usually have one person who makes a trip with that before we gather up the chairs and the tarp. Otherwise, we just get it all when we take up the tarp.
I too am amazed at the amount of stuff left on the ground. Appalled might be a better word. Reminders from the emcees and notes in the program help. Ultimately, we all have to be willing to do our share and a little bit more. It's really not that hard.
LuAnna