Those knots have their uses, and I do use several of them, but I have a very simple but very effective way of tying down loads withoug any fancy knots. I've used this technique for many different things, like securing lumber sticking out the back of my truck so it doesn't slide around. I hope this diagram I made and my explanation are clear enough. The picture below shows a box, and two tie down points (circles).
I tie the line to one tie down point (on the right), stretch it over the load (box) and a couple of feet short of the 2nd tie down point, I tie a loop in the line. I then run the line through the 2nd tie down point, back up and through the loop. The, I pull the line back toward the 2 tie down point from which it came and pull it tight. This allows you to pull the line tighter with leverage. Then, hold it where it passes through the loop and tie a double slip knot to secure it. If you are using a smooth nylon line, you might want to use a more secure knot. Using the double slip knot works great with sisal rope, or the "string" the box stores supply you to secure loads, and just a simple pull on the end unties it. I've never had one slip yet.
