I've been using climbing slings with my DD camping hammock for over a year now.
I basically put a sling (usually 5m one) round each tree and feed it through itself so that it tightens against the tree. I then attach the long end to the sling/carabiner that lives permanently on the hammock. This loop is made with a really short (0.5m I think climbing sling) that is fed through the hammock end. So what I end up with is this.
Through the long sling round each tree, hook the hammock onto one tree, unroll it and hook it on to the other one. Hammock up in under a minute.
If the spacing is wider than I would like I add more slings, if it's shorter I double them up or tie overhand knots in them.
The slings have little stretch and give a really comfortable hang.
Here's me lounging on a canoe trip down the Dee. The pictures aren't great for showing the slings in great detail but you get the idea.
Must do that again sometime