The Taut Line Hitch and the Trucker's Hitch are the most commonly used knot for this purpose.
If the taut line is slipping, make sure you tied the half hitch at the end in the proper direction. Also make sure you tightened the knot down enough. If it's still slipping, throw in a third turn on the inside and/or a half hitch on the outside.
Alternatively, you could use a Siberian Hitch on one end, and the knot shown in this YouTube video at 0:19 in (which I haven't been able to positively id yet). These have the advantage of being quick release.