Concentration is the ability to intentionally focus one's attention onto something. There are two general levels in this process: negative and positive concentration. In the first, one manages to eliminate distracting thoughts and pay attention to the subject at hand. In the second, one proceeds to focus increasingly more mental energy onto the subject.
These levels are analogous to the ways one improves the quality of sound on a radio. In level one, negative concentration, one fine tunes the dial in order to reduce the static caused by other frequencies. With static removed, the sound of one's desired frequency becomes much clearer and sharper. In the second level, positive concentration, one turns up the volume.
One way of strengthening one's power of concentration is to think of a word, colour - or any object for that matter - and to try to keep it within one's mind for as long as possible. The restless mind will attempt to replace the mental image with other thoughts. Use your will to continue projecting your desired thought onto your mental screen, revive the image again and again. If you find it difficult and persevere nonetheless, your ability to concentrate strengthens. Remember, strength is typically developed through resistance.
When skateboarding one often fails to perform optimally because - usually without one's awareness - the mind is bombarded with all sorts of thoughts irrelevant to the trick at hand: What's for dinner?; Who am I going out with tonight?; Do I look cool performing this trick?; I'm late, my mother is going to have my head!; Look at that attractive person sitting over there! Perhaps I should try another trick now?, etc. By developing concentration such 'mental static' is reduced, and one can skate with heightened clarity of awareness and increased confidence...