A couple of things that help for me –
1) take a walk, a lot! I have a desk job and generally follow the rule of 50 min desk, 10 min walk every hour. Your body needs to move to help it concentrate. If you do not have that flexibility, see if you can get a stand up desk or sit on one of those balance balls at your desk.
2) diet, diet, diet. ADHD brains are high performance engines and require premium fuel. Follow some of the diet suggestions on this website, lay off the white sugar and candy (if you are like me, candy is really hard to resist), and even go as far as trying one of those special diets. I have had really good success with Plant Paradox and The Zone Diet. Also, a good fish oil supplement is helpful. Diet remedies will kick in within a few days if you follow them. You do not need to be religious to them except you will find excess sugar will kill most of the benefits of any diet. Also, do your best to eliminate corn syrup.
3. If you still have trouble with brain fog after trying those above, you should talk to your doctor because you may have something else going on or need a medication dose adjustment.
Best of luck!!!