If it has to drive external components and there is some power involved, for simplicity I would consider a small latching relay circuit like this: -

http://www.the12volt.com/installbay/uploads/E33_Latching_Relay_2.gif

The "stop" switch is of course not needed and can just be wired short. You can use a P channel MOSFET in place of the start button.

If it has to drive external components and there is some power involved, for simplicity I would consider a small latching relay circuit like this: -

_{(source: the12volt.com)}

The "stop" switch is of course not needed and can just be wired short. You can use a P channel MOSFET in place of the start button.