There is a supplement called Ginkgo Biloba, which you can buy in chemists, health food shops and even some supermarkets, which is said to increase concentration levels and memory. Very good for a bad circulation, too.
dont waste your money on kinkgo biloba it has been proven not do do anything at all. spend your money on fresh fruit , too many people have been taken in by this.
Develop a series of exercises. Concentration can be developed by a concerted effort to practice.
If you spend 10-30 minutes per day doing something that demands maintained focus you may well see a gradual improvement.
I found that doing still life drawing at art school helped me develop better levels of concentration. I guess because still life drawing requires you to look very hard and long at inanimate objects.