How much do you like your stuff???
I am from Chicago. If you live in a large city, I would tip minimum about 100 bucks and I am not kidding.
If you buy them lunch, then I would tip them less - like 60 - 75. Also, tell them to save the table for last so that they have place to eat. Treat them like they are your own family.
A little goes a long way, and 100 might seem like a lot, but when you realize how much they can affect the outcome. Often they will be picking up other people's things on the same truck and working to get everyone's things delivered. Taking care of them gives you and edge over the other people on the truck.
After we did a cross country move, we ended up talking to the guy who was in charge of driving (he has the most influence on the move). He said that a little goes a long way - simple kindness like sitting down and eating with them, offering them drinks and snacks, asking them if they are doing OK. A cash tip seals the deal and protects your stuff! They will go to the mat for you.
Good luck.
Melissa