I've found the use of the bowline, prusik, and tautline hitch to be extremly effective when pitching a tarp. The bowline forms a secure loop that will not jam and is easy to tie and un-tie. I use this particular knot for one end of my ridge-line and tie-outs. I use a tautline hitch when stakeing out my tie-outs since it can easily be slipped to tighten or loosen along a line while able to hold under a load. With this same desire for ease of adjustment, I make use of two prusik knots to hold my tarp tight along the ridge-line since they too slide easily yet hold under a load.
See also: Winnie The Pooh - Can you tie a knot?