I second the Yoga- build some core strength, do you get plenty of healthy fats in your diet? Plenty of oily fish? Try and take a break from sitting every 15mins or so- change your position.
A bit health and safety this one but if you're at a computer and desk, have you set up your position correctly? If it's a sofa issue I think your sofa may be too low if you're getting knee and back issues (esp lower back problems) There should be a nice right angle position your body makes when you're say on the sofa - some sofas cause they're low down cause the bum to instead of being level with the knee bend to sink lower hence causing the potential problems mentioned.
http://www.nismat.org/ptcor/ergo
That's a link to a good set up for computer - however, that diagram is useful to show the way your body should be aligned when on the sofa