Theresa of Little Turtle Knits suggests in her stripey slipper pattern to knit the slippers then get a piece of leather or suede, put it on the floor, have the wearer put the slippers on and stand on the leather. Trace the sole of the foot, cut out the shape from the leather, put some holes around the edges and whip stitch it to the sole of the slippers.
I would imagine this would work with any pair of slippers really. I will be doing it for my kiddos this year b/c our new house has some hardwood and I don't want them slipping.
You could also use these~ http://www.knitpicks.com/Suede+2-Pie...SlipperB.html?