I use 2 kill switches, connected in parallel.
One is a simple microswitch that is operated by a small servo and connected to the channel 5 in my radio.
The other one is a normal radio switch, mounted in a Dubro cradle with a pin that comes out through the side and has to be pulled out to "enable" the engine. This switch is mounted near the engine.
Both switches have to be in the off position for the engine to start.