With a bit of Sugru, a few balloons and some kitchen knives, you too can enjoy the thrill of having your cheap radio controlled cars fight. To the death!
Step 1: The build
Inflate a balloon about half way then duct tape the neck to the car.
Take your balloon wearing RC cars and use some Sugru to bond a knife (or knives!) to the vehicles. (Sugru is great as it is tough and gap filling, & despite the strong adhesion you can remove it again)
You can see where this is going? Good stuff! If not, watch the video "Mk1 rc fight club" on YouTube: http://youtu.be/QbEZ4Kh9dS8
(Sorry for the dodgy screen grabs, blame the DRM. The video is in HD!)