[eevilspock]

This one says sitting straight is bad, that a 35% lean back is ideal: http://mashable.com/2011/05/09/sitting-down-infographic/

I have 2 blown lumbar disks, and for me at least, the 35% lean back, even if it means slouching on the couch, eliminates pain when I do have to sit for extended periods.

[molladair]

I work for a standing desk company (http://www.focaluprightfurniture.com/) that also makes "leaning seats" to pair with standing desks. The seats keep your hips open at a 130-135° angle - like the "lean back" approach, but while standing. I've been using the setup since I started and I love it. Definitely the ideal.

[wishinghand]

I switch every hour between standing at my desk and leaning on the Mogo stool you can buy from this company. Great alternative to standing all day (tiresome, sometimes painful) to sitting all day (bad posture, induces weight gain from being sedentary). And you can still wiggle around a bit with it. I keep some blocks under my desk to help stretch my calves with while doing either.