First of all, what are you sailing? Different boats require different techniques. For example, when I was sailing Sunfishes I wish I were thirty years younger again, just for that (okay, for the sex, too) the main technique to know was “Sail It Flat”. But classes of boats with more rounded hulls generally do better with more heel. In nearly any sailboat, you’ll want to find out (practice, practice, practice) what are the best angles of attack for pointing (for example, it’s not always best to pinch the wind and point as high as possible—it might sometimes be wiser to ease off a point or two and maintain more boat speed), and where the best weight placement is while reaching and running. In all cases it helps to avoid moving about in the boat too much. Practice working the boat (especially in light airs) with a minimum of body movement inside the boat. Pull anything off the deck and into the cockpit that you can to minimize wind resistance (including you and the crew).
Aside from “technique”, condition of the boat is crucial. Is the bottom slippery-clean? The centerboard / keel and rudder as smooth as possible (and also clean)? Do the parts all work properly? Are the fittings, halyards, sheets, cleats and sails in good condition?
After all that, you have to know how to ‘feel’ your boat’s performance and know exactly how you’re going to perform each maneuver you need to make, from roll-tacking, to gybing, spinnaker sets and recoveries (if that’s part of your SailBag), headsail changes (if you’re on a boat that has multiple jibs and sets) and you absolutely have to know your ‘grounds’. Know the wind patterns for your area at the time of year you’ll be sailing and the time of day.
Practice, practice, practice. That, and watching how the winners sail.