A forward lean of at least 45 degrees is most efficient because the strong gluteus muscles of the butt aren't able to contribute as much pedaling power when you're sitting more upright. This forward lean also shifts weight to your arms, so your butt doesn't get as sore. When riding, don't always sit squarely in the middle of the saddle. On steep climbs, crouching over the handlebar while sitting on the saddle nose maintains traction and keeps the front wheel down. Your knees should be slightly bent at the bottom of the pedal stroke, and, when viewed from behind, your hips should not rock as you pedal.
Most female riders prefer level saddles, but some like a slight nose-down tilt to avoid irritation.
A nose-up position can put painful pressure on the genitals when you're leaning forward. On a properly fitted mountain bike, you'll see more seatpost showing than you would on a road bike. That's why off-road posts are commonly 300 to 350 mm long. Don't raise the post above the maximum-extension line that's engraved on it. When you're seated comfortably, a plumb line dropped from the front of the forward kneecap should touch the end of the crankarm when it's horizontal.