use a battle switch and one of these

http://cgi.ebay.co.uk/Automotive-Relay- ... 1e6344df4e

or one of these

http://www.jaycarelectronics.co.uk/prod ... BCATID=754

The second relay is more expensive however it is far more reliable as it is a solid state relay (no moving parts). The lack of moving parts means that there is no chance of the relay fusing when it switches over (the spark that comes when you connect wires). I would not recommend a servo because it isn't an iron clad failsafe when radio contact is lost.

Boner, when it used speed 900s, used the solid state relay and a battle switch (battle switch to interface with radio gear and switch on the larger relay which directly controlled the motor)