I swear by one of those PG belly belts. Mine had a big elastic band and Velcro to fit it under my belly, every day. I tried it on as a joke in the maternity section - and just loved the relief it gave me. Try Target or BRU - I got mine at Burlington Coat Factory!s maternity/baby section.
Gah, until I started reading these replies I totally forgot that I wore a support belt towards the end (granted, only when working out, but still). I would look into one. It helped a bit with relieving the pressure on my back from just the sheer heaviness of the belly.
I should have probably switched to an exercise ball at the office since that was really what killed my back - by lunch I was just counting minutes until it was time to leave. If this is an option for you, I'd highly suggest at least trying it to see if it helps.
I'll second the chiropractor and support belt. If you don't have a footstool at work get one. I used a box of paper and had my feet up and shoes off everytime I was sitting. A pillow between my knees when I was sleeping was another big help.
Oh, and if you have a spa tub with jets, I highly recommend hot baths with the jets right on your lower back. It feels awesome at the end of a long day.
I am having the same problem. Sitting at a desk all day is the worst - even with my feet propped up on a case of paper. I have an excercise ball; it helps a little, but I get annoyed by people commenting.
Getting up and doing a lap around the office every 45-60 minutes helps. Standing still and sitting down suck - I'd rather walk around all day!