The Bowline knot is a great knot to use when you need to tie a knot and not have it slip. It is quite useful for mountain climbers. You can tie to each other without the threat of having a knot slip tightly over you. It is a great rescue knot as well. Personally it is my favorite knot to tie.
To tie the knot you first make a loop in the knot. Secondly you take the rope put it through the hole and around the main line. Finally you pull the rope back through the loop and sinch it tight.