Get LiPO alarm and you should be fine, they monitor the cells and if they drop below a certain level it'll beep. I don't have one, just don't keep firing after you start hearing/feeling the battery starting to strain to fire the gun.

Other than that you don't really need anything, don't shoot your gun in continuous sustained bursts, etc... If it seems like it would be more strenuous on the gun then don't do it.