Ooh ooh ooh!
I also work a sedentary job, but I've been able to maintain my weight; I think the fact that I work from home helps 'cause I have no co-workers to look at me oddly if I work out.
Big, big thing would be to see if you can stand up and do a lap around your area (walking) every half hour to an hour. I'd have to look it up further, but your body's ability to metabolize goes down a lot while seated. If you work in an area alone or rarely visited, try seeing about bringing in weights (5-10lbs) to work out your triceps and biceps while seated.
Another thing is that you can do is do leg raises from your chair. I usually brace myself on my armrests, or tuck my hands under my thighs and do 20-40 depending on the time and how tired I am.
I wouldn't recommend exclusively sitting on an exercise ball, but having one around for switching back and forth every hour or two would make for some nice variety. I tend to start out in my chair, then after my first 15 minute break, I switch to the ball, then I switch back to the chair after my lunch break, then kneel until my last break (will explain below), then usually sit in the exercise ball for the tail end of my shift. Again, might not be very practical. I make sure I kinda rotate on the exercise ball to get my abdominal muscles flexing. I've found a way to sit at my desk with it so that my back doesn't hurt, my legs are raised, and my abs are engaged, but I don't see that practical unless A.) you have your own private area, and B.) your desk is against a wall and not too deep and you can put your feet on the wall without damage to your desk or kicking cables--I know your said you're an IT consultant so I'm not sure what your specific working conditions are.
As far as kneeling goes, they do makehttp://www.officechairadvice.com/images/assets/reviews/wing-kneeling-chair-large.jpg, which might not be practical in your situation. I personally lay my backrest on the ground and kneel there. While doing that, I strap 1.5lb ankle weights on and raise my legs so that my heel hits my rear and they work out the backs of my thighs and get good circulation going.
I've seen some people able to fit http://image.made-in-china.com/2f0j00lCWEHpDJnwkP/Foldable-Mini-Treadmill-FT-F354-.jpginto their offices. I have an actual treadmill in my office but this looks cool if your desk is a bit higher; maybe do a half hour on it. It's cool how it can be folded, too. Doing a lap around your area every half hour to an hour, as mentioned above, would probably be more practical.
If you get the munchies at work, bring things like strawberries, bananas, cucumber and/or tomato slices (with some salt and pepper on them), nuts, and the like. That'll help curb your appetite while making sure you're not having to make trips to a vending machine or a convenience store. I get the munchies a lot so I make sure I have those things on hand.
If you tend to wear pants to work, you can wear ankle weights and walk around with them too.
And that's all I can think of for now, haha. This is something that I do a lot because this was my first sedentary job and I was afraid that I'd undo 10 years of weight maintenance. Recently, people have told me that I've lost weight, so maybe it's working better than I thought.
I mainly recommended things you can do at work because you can work out all you want outside of work, but those 8-12 hours of sedentary work can really kill your efforts outside of work.
EDIT: I wouldn't say to ditch the mini fridge as well as replace what's in it with punnets of strawberries and blueberries and/or other similar things that you like. Bring water with you. Lots and lots of water. Even if people look at you weirdly for pissing every hour.
There are different ways to lose weight for everyone; some people like to remove things from their diet and other people like to improve things by adding good things until there is no more space for the bad. I personally also ditched soda, chips, wheat (I kinda had to due to an intolerance, or I wouldn't have done it so readily) and most candies, and found good in other non-processed foods. For energy, I buy a bag of dark chocolate chips and get a pinch of them every once in a while.
If you find it difficult, one thing that helps me when I'm making a diet change is to tell myself that I've been doing whatever that change is for 6 months. It makes me take myself more seriously, and the best way to lose weight is to make lifestyle changes, not diet plans. When I had to give up gluten, it was much easier for me when I would tell myself that I had been doing it for "months" instead of days. It could be seen as lying to myself, but it helped a lot. I felt more confident in my abilities because I got that rush from already feeling like it was easy for me.