Climbers, Boy Scouts, and sailors may disagree, but the TRUCKER'S HITCH—highly adjustable and capable of piano-string tightness—is the handiest knot on the planet.
(1) Secure one end of a rope to your first tie-down point. Then make a quick-release loop between it and your second tie-down point. To do this, grab a section of the rope in your hand and twist it clockwise three times, then pull a section of slack line through it until you have a taut loop.
(2) Thread the loose end of the rope through the second tie-down point, then back through the loop you just made, and pull to tighten.
(3) Once you've got the line as tight as you like, clamp down on the loop with your thumb and forefinger, then finish the knot by wrapping a section of loose line around the taut line and pulling it through. To undo, simply tug on the loose tail of line to unravel the whole package. (Or, to really lock off the end, wrap the new loose loop you created around the taut line and pull it through.)