David and Herb, I've tried the talc and heard of graphite--the lock lubricant in oil type--being used but I don't think that either is a long term solution. The problem is that most squeaks are the wood moving on the nails and chattering is they move. Wood moving against wood noise seems to be less of a problem and that's what the talc or graphite will fix. This seems to go away with changes in humidity anyway.
Herb, your approach is correct in some instances. The only thing that I would add is that driving 2 nails at angles is a lot more effective than driving one that's "up and down" as the angled nails hold the floor from moving better.
But there are things like the screws that break off after they have been driven that hold even better. You can use these in bare flooring or through carpet. There are some other great tips in the magazine that I think are worthwhile for more unusual situations, like everybosy seems to have.
One note is that you might want to have some idea where wiring or electrical boxes or piping might be running in the space between ceiling and floor. Two minutes looking from the first floor could save you a heck of a lot more time and money as you drive a nail into the second floor. You can usually "follow" a wall or two between the floors to orient your location against areas of potential problems.