Para cord (550 cord, parachute cord, etc) works great and in a pinch, you've got strong, lightweight cordage handy. Small bungee cords also work pretty good, but prone to stretching.
As to exactly how to attach it, that all depends on what's comfortable, and what kind of tie-off points you have handy. I'll see if I can drum up some pics of how I used to attach my bedroll when I was still using a school-type book bag.