It's easiest to tip in cash. This way the money can be divided evenly and is guaranteed to get to who you are giving it to. Some moving companies have a place to tip when you pay the final price. If it's a credible moving company, this is also an option.
When giving the tip, be sure you hand the amount to each mover and thank them for their work. If you hand a lump sum to one worker, there's a chance they will pocket the whole amount. It's also suggested to give smaller bills you can divide easily. If you hand them a hundred dollar bill, it will be hard for them to divide.