get a new sending unit for the fuel pump.... or depending on miles... just replace the fuel pump... thats what i did... was a pretty straight foward install.... here is a how-to from fullsizechevy if you dont really have a clue how:

my fuel guage was really screwed up, and changing out the pumps helped (mine came with a new sending unit)

http://www.fullsizechevy.com/fscu/fuelpump.shtml

if that doesnt solve your gas gauge prob... you could allways check it off the list and go from there