In reply to Cadairmanuk: Bowline is a great knot for speed and ease of tying & adjustment, coupled with good strength. It's normally my first choice when tying a rope to a tree or boulder. However its Achilles heel is that it can be very weak when the two sides of the loop are spread at a wide angle (eg one rope pointing to left hip and one pointing to the right). Tying a half double fishermans (barrel knot) as a stopper will make it a lot safer but there are knots that are better suited to the task. One of the reasons that ML courses use the overhand so much is that it's much less 'picky' in how it gets loaded (as well as being really simple to tie). Other than using a rethreaded overhand around the waist you can make an easily adjustable loop like this:
-tie a fig 8 a couple of meters from the end
-tie a half double fishermans ('normal' stopper knot) above the fig 8 to make a loop
Should give you a loop that slides open so you can put it on but the fig 8 stops it getting too small. Works well for quick resizing/swapovers for the next one if there are a few of you.
Hope that's all clear as mud!
Tim