Fuel gauge sticking
 

I've got a 1998 Explorer, 4.0L SOHC with 120k miles. The problem is that the fuel gauge doesn't move as the fuel level changes while the engine is on (while driving). If I turn the key to the off position, then back on, the gauge will then show the correct amount of fuel in the tank at that moment, but won't change again until the keyswitch cycles back to the off position. Not a big problem for short trips, but for long trips you have to be sure to stop after a given number of miles to shut the engine off to allow the gauge to show a new reading. What could be the problem(s) and possible solutions? How can I troubleshoot?
Thanks!