Beanie, it's best to let the tank get down quite some way before filling it up. My dad fills once his needle comes off full (still has about 7/8 tank left) but it's OK in his old t-bird. He had my truck for a week and filled the tank after, then told me it seemed the gauge didn't work - just explorers being explorers. It actually says in the manual (and some servicing directions) that if enough gas isn't put in to make a noticeable change then it may not register for 50-100 miles, by which time you'll likely have used that much gas again.
On the other hand, I once had a corolla whose previous owner regularly used Lucas Fuel System Treatment, and the gauge in that took 15 minutes to rise after filling even from empty, as it had been gummed up by the solution. Eventually after a few months without adding the Lucas, the gauge acted normally again.
Having said that, you may just have a bad sending unit which would need to be replaced.