Skipping: Do what andre nickatina said. That advice is genius. To finish, once on the bike, try this jump, and then as soon as you've jumped, pull back like crazy with your front foot. You're trying to keep the pedals from turning and thus the wheel from spinning once you land. That friction of the wheel sliding (or spinning backwards if you are strong/have a funny gear) is what slows you down.

Skidding: Seriously, get your nuts on the stem. Skid with the same leg pattern as your trackstand (left foot forward/right foot back, or whatever you do). Just pull back on your front foot and push like hell with your back foot. Once you get the feel of it, unclip your front foot so that you don't use it at all. You don't need it.

Unweighting is literally trying to put as much of your body over the front wheel as possible. You normally sit in the saddle over the back wheel, so putting your nuts on the stem is literally trying to ride like the stem is your saddle. Shift that weight!