Make them carry your weight: gravity is a truly powerful force – make sure you use it as much as possible in your jiu jitsu. When on top and past your opponents legs, use your weight and center it over your opponents head and shoulders and within a very short period of time the hard physical work of holding that weight up will break him and make him take risks he would normally never take in order to relieve the pressure. Be ready to capitalize on those risks and unforced errors. Experiment with ways and methods of maximizing the felt pressure of your weight. You will find that getting off your knees and into tripod posture often greatly increases your felt weight and allows you to direct your weight with great precision through your head and cross face shoulder. You don't have to BE heavy in order to FEEL heavy (though of course it helps ?) Learn to use your weight to advantage and you'll find that even the toughest matches become easier as time passes.
