The information below is based on receipt of good service. If you feel you aren't receiving good service, don't feel compelled to tip. Tipping should be reserved for movers who do their job right. If a mover goes out of their way to accommodate you, then tip accordingly.
1. Provide cold drinks, coffee or whatever is suitable to the weather and time of day. Also provide food if the move takes a while and extends over the lunch period; pizza, sandwiches, something quick they can eat while on the move.
2. If there's only one or two movers, tip between $40-60 (to split if there are two) depending on how hard the move was (stairs, heavy boxes, awkward items, etc...).
3. More than one mover, tip $20 per person and give all the money to the head mover or supervisor so they can divide it afterward.
4. If you received absolutely amazing service, tips can reach up to $100. But the movers need to be really good, really helpful and do something beyond the call of duty, such as helping with last minute packing, taking the time to carefully wrap fragile items, or in my case once, helping me catch an escaped cat. That's definitely the sign of a really good and caring mover.