The other day, while walking around at a local greenfestival, I spotted a guy wearing something that looked like a fanny pack with very long straps hanging off the sides. Later, I spotted the same item at a booth for something called Nada-Chair, which, true to its name, is not a chair, but a simple, lightweight device that supports your lower back whether you're sitting in a chair or on the ground. I tried it, and the experience was rather amazing, not at all what I expected from watching other people trying it out. You wear the main part of the Nada-Chair around your waist, and then tighten the straps around your knees. This pulls your lower back upright with no effort, correcting your posture, and taking pressure off your spine. It comes in seven different models for adults (some fold and zip into themselves to become small carrying bags, some have larger built-in packs for carrying gear or cold drinks). There's also a kiddy version to help your toddler play comfortably on the floor or while finger-painting at a table.

The premier model, the Back-Up, has been used by astronauts to reduce micro-G induced back pain. I'd put it to more mudane use - to keep my back straight while sitting in an office chair all day.
Prices range from $25.00 to $65.00.