Yeah, starting a desk job was the shits and the pits for my lower back as well. When I started working in an office after college, I had some pretty bad and frustrating back pain that eventually lasted many years.
Some great advice given already, so I'll just mention what worked for me:
-I got a new height-adjustable chair and set it to the lowest possible setting so that I never had to hunch over to type or look at the screen. It also has fairly good lumbar padding, so I think I tend to maintain decent posture, even after a 12 hour day.
-I spend as much time out of the office NOT sitting or standing...at home I read, draw, surf the internet on the floor, lying on my stomach.
-I make sure to stretch out the hamstrings, as mine get pretty tight.
-Of course doing quality exercises (squats, pulls, etc), which I know you're already aware of.
So far so good. Haven't had significant pain for a good year now, and actually the back is feeling pretty darn healthy nowadays.
I've been looking into getting a standing workstation as Garrett mentioned. And as a long-term goal, that cattle and coffee farm in Costa Rica is sounding pretty attractive