ummmmmmmm..........by using humbuckers?
outside of that, try the following.
1. clear up any grounding issues with the building's power.
2. clear up any grounding issues with the guitar.
3. make sure there are no dimmer switches on the circuit.
4. turn off the CRT monitor while you track or get an LCD/plasma monitor.
5. shield the inside of the control and pickup cavities.
6. find a sitting/playing angle that seems to minimize the noise.
7. use a double shielded cable. the monster 500 is actually pretty good for this. this took care of the last 20% of the noise i was getting from my single coil strat, tele and PBass while recording DI.
but really, you can try to mitigate it, but single coil pickups are always going to have some noise. that's just part of what they do and why humbuckers were invented. you can always try noiseless pickups and such, too.
cheers,
wade