As mentioned by the others, your modfet drive circuitry leaves a lot to be desired. If you only want to turn the motor on or off at a slow speed, it may work. The gate of the mosfet looks like a capacitor, the bigger the mosfet, the larger the capacitance. Coupled with your 1k resistor, the gate capacitance form a RC time constant, thus the mosfet will take a finite amount of time to turn off - in your case this is probably too slow so you exceed the allowed dissipation for the mosfet and thus it dies. I would suggest you do some research on the international rectifier website and also On semiconductor had some good app notes on motor drives. In short, the general idea is to turn the mosfet on and off as fast as possible to minimise the dissipation.

Once you've addressed to drive issue, then there is the protection of the mosfet. Current limiting is always a good idea. I'm not sure of the size of the motor, but you may require something more substancial than just a diode across the windings - the bigger the motor the more energy you have to contend with.

Also the choice of opto is critical - what sort of switching frequency are you using?

You've entered the realm of power electronics - make a mistake and you get smoke!