Having the best ergonomic chair possible is the first step. Allowing yourself to sit with your shoulders relaxed and pulled back, sitting up tall, and having your forearms parallel to the ground, not reaching up to your keyboard will help. Setting the monitor and desktop at eye level is important too so you don't hunch over.
Next, ideally, you should try and move every 20 minutes. I know that's tough, but having something like a fitbit that you can set reminders on will help encourage you. If you have space you can do certain exercises that will really help. Thoracic bridge stretch and grok squat stretch are very useful. You want to try and keep everything loose so your muscles aren't pulling you foward themselves.
3rd, will be trying to exercise when you're not at work. It takes 1 hour of exercise to help combat 8 hours of not moving at your desk. Try and target whole body routines and focus on your glutes/legs because they're taking the most abuse at your desk, and could indirectly be pulling your body forward due to tightness.
I'd be happy to offer more suggestions later.