Simon's 7 point plan:

I) Use a noise gate
(this will only stop noise when you are not playing anything)
II) Avoiding ground loops
III) using humbuckers as opposed to single coil PUs
IV) do not play any where near fluorescent tube lighting
v) turn of all computer monitors when playing
VI) use well shielded or balanced cables (if possible)
VII) shield the inside of your guitar (copper / nickel foil)