The half-hitch is really a cornerstone knot that forms the basis for a multitude of other knots, and so you should take the time to truly master it. By itself, it is not particularly reliable or "slip-proof", but it can handle light loads when tied properly.
Knot Video: How to Tie the Half-hitch Knot
Knot Tying Instructions
Step 1 - Hook your rope around your post, tree branch, pole, or whatever.
Step 2 - Cross the short end under the long, main length of the rope.
Step 3 - Bring the short end over and down through the hole between where the rope crosses and the pole.
Step 4 - Push the knot to the pole and pull to tighten!