Mark Thomas of the NSSF talks with Bob Whaley of Gunsite about slings for the "modern sporting rifle"*. They discuss both 1-point and 2-point slings. It gives some good stuff on what to look for in a sling.
Slings don't have to be expensive. I just paid $20 for a decent 1-point sling at Wally Word that is actually pretty decent.
If you are good with craft-oriented stuff, read through this thread on Arfcom. It gives instructions on making many types of slings on the cheap.
*AR-15 or M-4gery