We bought a slip cover for our sofa. Some friends gave us their old couch and it had an ugly flower print on it and so we bought a cover at Wal-Mart for it. Ours has elastic all around the bottom so that it stays on better and covers the whole back of the sofa. My MIL bought one for their couch after she liked ours so much, and got it from JCPenney's, but hers didn't cover the entire couch, and as a result, it doesn't stay on and ended up being a pain in the butt. I like ours because it's washable - I usually end up washing it every 1-2 weeks cuz of the kid, pet, spills and whatever else happens to it, but love the fact that I don't have to worry about ruining a good sofa because something gets spilled on it.
My suggestion? Look for one that covers the entire sofa, loveseat, chair or whatever and is definitely washable.