The safest switch is a blind plug. Take an extension, cut the negative wire, put a female connector of your choice (there's some JR connectors that come with a tab if you want extra security) into that place, the end going to your battery as pin 1, the other as pin 2 (or vice versa). Then get a matching male connector and just short the aforementioned pins on that. Need to turn it on? Put in the plug! Need to turn it off? Pull it out!

I also like electronic switches. Many companies offer them, hobbyking has them for cheap under the TrackStar brand, SkyRC has one, Futaba has one, there's a Protek one as well iirc.