As said above, make sure you keep moving and move your neck and spine in all it's possible directions.
Do you have anything you could get up for regularly such as getting up to get something from a printer or make a drink etc... Make sure you move out of your seat and stretch at least once every hour and say every half hour check your posture and make sure you're sitting up straight and straighten your back and neck out (flex the other way if you've been sat in one position.
You could set a reminder on an email calender if you have that on your computer.
Definitely get a workstation assessment Make sure your monitor is at eye level (I have mine on a box to raise it), your feet are comfortably flat when sitting (if not get a footstool) and your spine is straight when seated.
You can get special cushions like a lumber roll and seat wedge to help you sit in the correct position.
Make sure your keyboard and mouse are in comfortable positions.
You can get document holders to stop you having to look down at something and if you are on the phone a lot and find yourself reding, writing or typing at the same time then you could get a headset for the phone.
There is a good checklist here...
http://www.openerg.com/dse/mainform.html
Also some good info here from the HSE...
http://www.hse.gov.uk/msd/dse/