Without getting too political do you mean a momentary switch like this one?:
http://www.tlc-direct.co.uk/Products/MKK4878P.html
Depending on how your garage door works you might need 2 - one to open it and one to close it, or a different kind of switch, but if you stop pressing the button because you've fallen over/passed out/lost interest/forgotten what you were doing etc, the motor should stop.
Adam, October 2009