Other than good fitness and good luck, a proficient, fast rider relies on good line choice and momentum to roll on past other riders. This ability to ride light and pick the right line will keep you fighting forward faster and diminish your fading off of the back (with a severe case of Monkey Butt).
Tire Line Choice
Choosing the right line is crucial to your comfort in a long ride. The effects are exaggerated, positively or negatively, in low-gear/steep-climb situations when picking your way through the rocks. In this scenario, nearing the end of a ride, your body and mind are fatigued and a little slower to react.
Therefore, you can spin out or get bucked out of the saddle more easily, forcing you to dab and/or stop completely. This is where line picking comes into play.
As your wheelbase swings through a low-speed turn, the front and rear wheels track different arcs. As you are under power, the majority of your weight is on your seat and pedals, and therefore upon your rear wheel. Any bump force encountered by the rear wheel meets much more resistance than the front. So in knowing that, choosing the line of least resistance is your goal.
Let's put this into motion. Cranking your way up a windy, rocky hill, ride the front end light, letting the suspension do its work. As you encounter small and medium sized rocks, steer your bike in a way that allows your rear wheel to slice around or between the rocks.
The front can be steered around or wheelied over the obstacles, but the objective is for the nasties to pass on by your rear wheel, never bouncing you around and breaking your rhythm. Practice this and do it all day and you will both feel and see the difference in your riding as your overall speed increases.
Now comes the fun part. You have been riding for quite some time and are getting faster and have more ability and power. Something to focus on now is using your momentum to your advantage. This becomes a combination of picking good lines (in corners and technical sections), riding in higher gears, and using your body's ever increasing power.
Instead of attacking that hill by downshifting, upshift and build your momentum. Try to ride every section of your local singletrack in one to three gears higher than you usually use. Run wider arcs through corners with more entrance and exit speed. Run lines that will carry more speed and drive you through and around obstacles, not over them. Sit down more and power through singletrack in a higher gear while sitting a little further off the back of the saddle.
The more your bike stays in contact with the smoothest part of the trail, the faster and less fatigued you will be. This aggressive approach to riding takes a little more energy at first, until you become accustomed to it. Once your style is up to speed, you'll actually use less energy as your overall speed increases. And you have much more fun with your riding. So get out there and RIDE IT!