For 3.3V microprocessors, what's the best way to power the gate of a MOSFET?
Three options. 


Use a mosfet with a VGS compatible with 3.3V.  Typically known as a logic level mosfet. 
Use a simple npn transistor as a switch to drive the mosfet at a higher voltage. Logic would be inverted.
Use a dedicated mosfet driver IC.