Is there a device I have to use to drain them? Or just leave the battery in the car with it on until it drains?
You do not want to completely drain your NiMh every run. You want to stop using it when the RC slows significantly and then recharge after it cools. Completely draining and recharging them every time will kill the life span dramatically.
Whatever it is I just typed... could be wrong.
Is there a certain voltage I should not let it get to?
What kind of charger you using. With mine after a run i will usually run a discharge / charge cycle. I have my charger programmed to bring them down to .9 or 1 volt per cell (cannot remember). I find this way i get accurate info on the state of my batteries by the final mah reading. Its not necessary to do it i just find this system works well for me. I have actually been able to save a few batteries this way as i seen a few times where the charge was very low so i set the charger to 1 amp and cycle charged a few times until the battery came back to full charge
I've just got the Traxxas charger. I only have the rechargeable battery for receivers and the ez start.