I think that the cop who arrested me was abusive. After he handcuffed me, he pushed me into the back seat of his cruiser and smashed my head against the door frame. Is there anything I can do?

Well, if that's all he did, then you would be better off not to pursue it. After all, most people do not like being arrested, and complain about the smallest matter that comes to mind. Many complain about the cuffs being too tight and being pushed around, but that does not equate to being beaten for no cause while under restraint, or kicked when on the ground or otherwise being grossly abused. If you have been grossly abused, then you should contact an experienced attorney who can help you obtain compensation for your injuries. But, remember, you must be injured to get compensation and mere bruises or being handcuffed too tightly is not going to be sufficient for an award of damages.