I have to agree, you should be only lightly on your seat. If your seat is glued to the seat of your bike, it will be unpleasant at best. Also, you need to be able to shift your weight while you go through the trail.
Roadies are planted on their seat, they are on a smooth surface. We aren't.
Use your legs to take up most of your weight - yes at first it will be hard but eventually it will just be natural. I spend the majority of my time on roads made from river rock being spread on them and pounded down by traffic. The hills are very steep as well - if I don't shift my weight, I will either spin out or flip the bike. There are books on MTBing that actually recommend removing your seat till you learn not to rely on it.