I think that short of a mathematical "formula" (which may not respond well to windshifts), the basic principles are:
If lifted, stay on it.
If headed, tack, it becomes a lift after you've tacked. So keep watching your compass.
In general, absent windshifts, stay on the tack which takes you closer to the mark than the other tack. But don't let the mark get abeam too early, or you risk overstanding and wasting distance.
It might be useful to picture a "cone" emanating from the mark, each edge of which is about 45 degrees either side if the prevailing wind direction. you want to remain inside that cone so as to avoid overstanding. The cone starts wide, then narrows as you get closer to the mark. While doing the stuff above, when you get near the edge of the cone, or are in doubt about if you're near the edge (which would be the "layline"), it's time to tack and dig back in towards the center of the cone, watching for lifts and headers as you do.
Of course, if the permanent wind direction (as opposed to just back-and-forth oscillations) shifts, then so does the "cone". That's the unpredictable fun of beating upwind.
Last edited by nolatom; 05-04-2010 at 11:01 AM.