You have to find a balance. Sit on the nose of the saddle and keep your elbows bent. This way you can't pull up on the bars. I find if I drop my shoulders and try to put my centre of gravity through my hips, and the effect is a sort of hunched over position that throws as much weight downwards through the seatpost and chainstays rather than power into the back wheel.
Gear selection is a bit hit and miss, but you need a low enough gear to be able to turn it without lifting the front wheel with a power wheelie, even better if you can learn to be subtle with your rear mech and change down on the fly. Too high a gear and you will produce way too much torque and the wheel will break away on a loose surface.
It may seem obvious, but you also need to plan ahead and do all your changing down before you grind to a halt, because making a smooth transition into a hill means you can keep momentum, and shift weight simply to keep the bike moving.
A woman at our local club makes this mistake every time. If you get stuck behind her there's always a grinding to a halt, a struggling with gears, thirty seconds spent clipping back in to a pedal that's now in too low a gear and loads of apologising. Subtle shouts of "Change down NOW Xxxe or you'll grind to a halt again and hold everybody up!" never have the desired educational effect......