Void meditation is perfect to improve focus and concentration.
Just concentrate on an object, it can be anything; put everything out of your mind and only think of the object. Do that for 5 mins with your eyes open at first (doing it with your eyes shut at the beginning will sometimes put you to sleep) in the morning and at night before going to sleep.
Other meditations can be found on www.joyofsatan.org - just click on "Meditations".
