This is not another text talking about the importance of exercise. Actually it is a little bit, exercising is an important part of maintaining your brain working well, but it's not the only thing. One of the most effective activities you can do to train your brain is meditating. It works for your brain the same way that lifting weight works for your biceps. With only 5 minutes of meditation each day, your concentration is going to get much better.
The first benefit of meditation is to get you out, for a moment, of your infinite list of worries, problems and tasks-to-be-done. Just breathing and paying attention to it really get you some peace. Secondly, when you meditate you're forcing your brain to stop thinking; you're keeping track of your breathing and doing all you can to stay completely there, not on your past or in your future, only there and only breathing. This act of forcing is what gets your brain more active, more concentrated. Believe me: meditation remodels the physical structure of your brain. So if you can't get your mind empty, that's ok, just by trying (but you have to try hard!) you're already achieving your objective. Actually, most of the people can't achieve the state of complete mind emptiness. The really important thing here is trying.
If you're more interested about the subject, I recommend go through the work of a psychologist called Kelly McGonagall. Her blog has lots of videos and articles to help understanding how to meditate, how it works in your brain, what you can expect to achieve by doing it and way more subjects related to getting your brain a better treatment considering nowadays lifestyle. Her books also deserve attention.
There's another interesting thing you can do to feed your brain: play mind games. Seriously, a bunch of simple games can get you more benefits than you imagine. A great platform dedicated to this purpose is Lumosity, an online tool developed for neuroscientists. It brings a whole program of brain training based on what you want to improve, and it updates itself based on your results during the training. The free program is limited, but it still works, and the paid one isn't expensive. Like Lumosity, there's: BrainTrain, Neuronation, Fitbrains, Cognifit, Brain Metrix, Memorado, and others. Find the one that suits you better.