Recently we were asked about the function and use of leech telltales on a mainsail. Leech telltales allow you to gauge the amount of twist in the leech of the mainsail by indicating the airflow off the leech at their various heights. You generally want the telltales streaming straight back, but when you're trying to go to windward and point as high as possible, you can get a few degrees more out of the boat by trimming in the main tighter. When you trim it too tight, the top section of the leech of the sail cups to windward, and the telltales there begin to stall and droop from the disturbed and inconstant air flow from the cupped leech. When this happens, ease the mainsheet out a little until they being flying again and re-establish proper air flow. As a good rule of thumb for the best upwind performance, you want the top two or three telltales flying 2/3 of the time and stalling 1/3, though this proportion is inexact and hard to maintain precisely as the boom surges up and down with waves and puffs. Next time you're sailing upwind, find this threshold -- trim the sheet until the leech closes and the telltales droop, then ease until they stream, then trim, etc. so you see the difference. And of course, "when in doubt, let it out."
Once you get the leech profile set up for the given wind conditions, you can increase pointing ability further by pulling the traveler to windward (not necessarily to the windward side, just to windward). When doing this, the leech profile will stay the same, but the angle of the sail to the wind changes and helps the boat point a little higher. Over-trimming the traveler to windward will cause the whole sail to stall, not just part of it, and you'll see your boatspeed drop. You can correct by dropping the traveler and bearing off again, playing that stall threshold as well until you develop the feel for it.
In raceboats, mainsheet trimmers constantly play the mainsheet and traveler, adjusting one or both frequently in response to changing wind conditions and the feel of the helm. On a cruiser, this might be a little obsessive, but when you learn what it looks and feels like when it's wrong, you can make it right more often and get more out of your boat.