I use a Niff-T-Seat. It's a great little seat. It has no backrest, so if that is important to you, then I would pass, but I've literally sat for hours deer hunting with this thing and it was more comfortable than I imagined it would be. It's also very easy to turn (quietly) and manuever to make shots you can't normally make on a seat that has a static/rigid base.
Last season when I got it, I simply used the belt attachment that came with it, so it hung off my waist behind me; this worked well.
However, I bought an Eberlestock Mini-Me backpack in recent months, used it on some scouting trips and while dove hunting. This pack has compression straps, so what I did was just put the straps around the seat and lash it to the outside of the pack. Found that this worked really well and will be using this setup when our season starts (less than two weeks! yeah).