If a track is steep and gravely I oftten find it best to lean BACK to prevent the rear wheel spinning too much (by causing more force between the ground and the back wheel and thus greater traction). This of course leads to a greater tendency for the front wheel to lift off the ground, but I have gone up some quite steep gravely slopes this way which I don't think I could have if I was leaning forward, and I don't particularly recall any problems with the front wheel lifting off the ground. I am not dissagreeing with what other people have said about leaning forward, just adding another dimension.