Providing food is absolutely key on the day of the move but I only consider that as a way to get them through the move itself. Thanks comes after, but that can be food too. My friends moved to a lovely house and as soon as their new BBQ was set up, all movers were invited back for a grill out.
Seconding Danielle's motion, I'd also include a note of thanks for each mover at the second gathering.
Or a dinner/movie night, if you're cookery inclined. Oooh, I just noticed the note about the weather, I hadn't considered inclement conditions. Might be nice to put together a small package of cocoa/cider + treats for them to take home.