Shadowboxing (see video on same subject for more info and demo)is just practicing your techniques in the air. You can do prearranged patterns or improvise your own. When you start out its best to do simple combinations that you've been taught (to make sure the combinations work together) but as you advance its best to start making up your own combinations and variations. At this point you will be improvising which is important to feel comfortable doing because if you ever do have to defend yourself real fights require you to improvise because like having a conversation with someone you can't plan out ahead everything that will be said because you don't know what the person your talking to is going to say, the same is true in a fight you need to constantly adapt to what your opponent is doing. When you do shadowbox it should be done at all levels of speed, power and exertion. For instance, if your max speed is throwing four strikes in a second, then most of your shadowboxing should be spent throwing one or two strikes a second. This way you can work on your combinations, balance, body mechanics, relaxation etc, to really perfect your technique. Only doing the three per second when you can do them perfectly and relaxed (if your making mistakes your going to fast) and only rarely doing the four per second. If you want to develop speed what will make you faster is good body mechanics, relaxation, clearing your mind and repetition. The risks of doing your max speed too much is greater chance of injury (pulled muscles, hyper extended joints.) Also at max speed it's easy to get sloppy and you will tire out quicker, not allowing as many repetitions (the repetitions is what you need to get faster). Learn to use all of your techniques. Don’t limit yourself to boxing techniques (unless you’re a boxer.) Shadowboxing should include strikes, joint locks, throws, sweeps, defensive actions, and footwork, everything in your style. At first you might visualize an opponent most of the time to make sure your combinations make sense, at other times you can work on not thinking and just clearing your mind and allowing techniques to flow this is more the mindset you want when actually fighting or sparring (this creates a form of moving meditation the same as practiced by shaolin monks)