You need a surface with decent traction...don't want the front sliding.
Another possible reason for sliding the front is not getting enough weight on the front wheel. You need to slide as far forward as possible, squeeze the bike hard with your knees, and only use 1 or 2 fingers on the front brake. The hardest part for me was always remembering to let go of the brake when it got up near the balance point.
Just as wheelies are easier to initiate going uphill, stoppies are easier to initiate going downhill. Be careful though, because of the fact that you start closer to the balance point when going downhill, you get there a lot quicker and it is easier to flip over the front.