I use webbing tape purchased from ebay. I seem to remember it was sold in a pair as 'canoe straps', complete with a metal cinch buckle. Came in well under a tenner with more than enough length to hang my hammock.
I cut a meter off each tape close to the buckle, put some further bar tacking on the buckle to strengthen, and attach to each end of the hammock. I then stitched a strong loop at the end of each length of the remainder of the webbing through which i passed a karabiner.
To erect it i pass the strap round the tree, clipping the Karabiner back onto the strap, then feed the loose end of the strap through the cinch buckle which is attached a short distance from the end of the hammock, and cinch- to both centre the hammock between the trees, and obtain the correct amount of tension.
What you get with this set up is something incredibly quick to fasten, non stretch, completely adjustable, no knots, and the buckles act as affective drip rings. All for less than a tenner!
Steer clear (if tempted) from the olive rope sold in outdoor shops (often called purlon)- it stretches far too much, meaning you have to highly tension your hammock if you want to avoid the ground. In turn, this results in the hammock squeezing you at the shoulders all night...
This is where i got the straps from (albeit a while ago). The buckles have a 900kg breaking strain, which is more than enough even with the additional multipliers you get in a hammock type situation.
http://cgi.ebay.co.uk/3-5-m-x-2-HD-...MasksSnorkels_Flippers_SM&hash=item2a02b64214
Nick