One thing is performance bonuses for exceeding a certain number of units per week, but in your case it could be for exceeding a certain number of hours per week/month.
My wife does medical transcription. The offshoring of these jobs has dropped the rate inside the USA by 40%. The get paid an extra 1/2 cent per line, if they exceed 1,200 lines a day. That is there performance bonus.
So, you could offer a perfomance bonus when their weekly or monthly hours exceed some number. If they exceed that number you can pay them an extra 50-cents an hour. Maybe even a whole dollar.
"I am wracked with such hearty guffaws that in addition to rolling to and fro on the floor, my posterior has separated itself from my body."