Check for the following:
--Low coolant
--Thermostat stuck open/closed
--Poor contact at temperature sensor
--Inaccurate reading from temp sensor.

You'll probably need a scan tool to effectively check the bottom items.
If you don't find anyting in the first two, you could try replacing the
temp sensor yourself.  That's probably about the same cost as taking it in
to have it checked.

Also, let me know what year and model your car is.  That will help me give
you info about likelihood of different failures.