I second the suggestion of a trucker's hitch. A ratcheting knot can be useful in this situation, but IMO what you're really after is a mechanical advantage. The trucker's hitch gives you a mechanical advantage by using a loop in the rope as a pulley. There are a number of ways to make the loop and it doesn't really matter which one you use. This video gives a pretty nice explanation.

https://www.youtube.com/watch?v=raTYwsfnsXg

Once you cinch the load down using your mechanical advantage, you can use any number of knots (ratcheting or not) to secure it.