Taut Line Hitch
Notes and Resources:
Good knot for taking whatever you've got on a rope (e.g. a limb), and tying it off to some post.
Animated Knots has this entry for both the Taut Line Hitch and Midshipman's Hitch, along with an explanation of the difference and history. Either one is perfectly suitable in bondage applications; the Midshipman's Hitch is slightly more secure, so if you don't know either, you might want to learn that one. On the other hand, the Taut Line Hitch is very slightly easier to tie, and much more widely known; if you're already comfortable with that, go ahead and use it.
The thing that makes this such a good knot for tying a bound person to a fixed object is that (if tied securely) it won't slip in response to their struggling, but can be adjusted if their attachment needs to be made tighter or looser during the course of play. The trick to adjusting it is to grab the knot with one hand, and then with the other pull on the rope that goes straight through the middle of the knot, right where it exits. Which side of the knot you pull from determines whether you make the rope tighter or looser.
To allow maximum range of adjustment, tie the knot halfway between the post and the person you are tying. Try to keep tension on the line while you're tying it; it's very hard to tie this knot well in a slack line.
When tightening the final half hitch, be sure to hang on to the first two loops with the other hand, or they'll run away from you toward the post. You need to get everything well-tightened before you let go, or it will start sliding before it ever bites down and sticks. In very slippery braided nylon/MFP, you may need to take 3 turns through the initial loop instead of only 2, for additional friction; however most problems with sliding come down to not tightening the knot well.