Personally I'm not a fan of the A-frame way of setting up a tarp.

When I do set it up that way I use my trekking poles to suspend the ends regardless of if I use a ridgeline or not. It's nice to have and adds strength but it's not always possible depending on where you are.

Assuming I'm sleeping and not trying to sit out some nasty weather, I'll set up my tarp as compact and sturdy as possible with as little of it up in the air as I can manage. Because I use a waterproof bivy bag, that can mean the lower part of my bag is exposed to the elements a bit as my tarp is only 5x8.

For a larger tarp, I like this way:
http://www.youtube.com/watch?v=MkBeZqXU4zk
but a guy line or two would be added.