I usually find driving 400 miles does the trick.
I'll get me coat...
Does it read the same all the time, or does it vary?
If it varies it may be a loose/corroded connection somewhere between the sender and the gauge (probably at the sender or gauge rather than somewhere inbetween). If it's always high then maybe the sender has had it.
There's bound to be someone on here who is more help than me.
Good luck!