My father worked in construction and, when I was a kid, he showed me that if you wrap up a small stone in part of the tarp, then tie a knot at the base of the tarp (under the stone), it works great for holding a tarp in place and making secure mounting points. He used to use this for securing tarps over loads in the dump trucks.

You can see what I mean on here:

http://practicalsurvivor.com/?q=node/28


I got so used to doing it that way I've never used anything else.

Edit: Ah, I see it mentioned in the first post.