How can I shield cables from interference?

The idea you mention is a good possibility. You have to solder a bit of wire from the shield to the ground pin of your powerplug which goes into the wall. The end , which goes into your gear, you'll have to leave open. This works only however, if you have a properly grounded line.
Besides, never run pcs in long parallels with your other wires and if you have to cross other wires with your pcs, try to do this at right angles. As set up like this, mostly helps a lot.

What about running a separate ground cable from the shielding near the male termination, instead of removing the plug and soldering to it?

Sure, should be no problem as far as I can see. I just find the first solution a bit more simple and less wires hanging around.(-:

Twist the power cable...several turns per foot.

When you install wiring in your house walls it is also a good idea to twist, although it doesn't look so neat to the building inspector.

I guess you'd have to hack off the male ends of your power cables and install a new one in order to do this, right? What kind of solder would work best for a fix like this? I'm hunting for the right kind of techflex to do this with - one of their reps told me only the conductive nylon would work for this kind of application...