Driving a servo with MOSFET
You didn't mention how much current you require. 
Here is a quick guide -- 

For most switching applications the important parameters
  are the voltage rating (BVdss), the maximum drain current (Id(on))
  and the gate turn-on voltage.

For a 6V battery you want a breakdown voltage at least 6V. Make this
  a bit higher in case switching produces transient voltages. Since 
  the majority of FETs have voltages of 20V or greater this should
  not be a problem. Choose a 20V or 30V FET.

Choose a maximum drain current above what the servo requires. 
  The maximum drain current is usually limited by the thermal
  performance of the system not the device. How much current do
  you need? How large a device can you use? Do you have room for 
  a heatsink?

To use the FET as a switch in a 3.3V system you want a logic level
  device. This will insure that the device is fully on (lowest 
  on resistance) at 3.3V levels.

For circuitry I will usually put a pull-down resistor on the
  gate so that the gate is never floating. For some applications
  I will place a zener diode across the gate for transient protection.