1.) the lego RCX trick wayish thing... get a 5v motor. connect the motor to a switch. have that switch connected to another power source(12V) & the 12Vmotor. so basically your 5v motor will mechanically operate the mechanical switch to operate the 12v circuit(motor).
2.) must you use full 12V? a 12V motor will run with only 5V. not as powerful, not as efficient, and i think not really good for the motor? maybe not.. i dont know about that part.
3.)stepup transformers may work.. but id worry about the amps involved. never really tried using one of these before...
E=RI so if you double E, Iwould become double... i think.. ive never learned anything about electronics.. all my knowledge comes from trial and error.. someone clear up what im trying to say, thanks.
http://www.williamson-labs.com/basic-ele-pt-2.htm
near the middle of in the posted url above, there is a simple diagram of a step up
howstuff works explains too
http://science.howstuffworks.com/inside-transformer.htm
but they explain stepdowns. think opposite of that. less windings on power source side, more windings on output.
sorry if im orribly wrong. yeah, someone needs to help me explain and correct me if im wrong.