I slouch when surfing on my desktop, but not when writing. I think it's the different arm position when typing that reminds me to sit up straight.
I rarely write at my desktop now. I usually use my husband's work laptop, and sit either on the sofa with my feet up, with a cushion and a large hardback supporting the laptop on my lap, or sit on one of those faux leather cube/footstools, with my back against a chest of drawers (and a cushion) and the laptop on a piece of board that is balanced on a couple of plastic boxes, with a laminator to counterbalance it. Both feel quite comfortable for my back. I do pay attention, as it's usually back and neck problems that cause my migraines. First sign of one coming on, and it's yoga DVD time, which stops it 90% of the time.

Don't forget to stretch regularly, fellow slouchers!