For me, pacing is the hardest part of writing a book. The problem with pacing is that it differs for each book, and depends heavily on the actual plot. Most writers know to avoid a plot that moves too slowly, but a too-fast paced book can be just as bad.
Planning can help to plan the pace of the book, but when writing you need to be conscious of the pace at all times. If an action scene seems too short, it probably is. When describing action scenes, constantly be asking yourself if you've mentioned everything you need to, and if you've described the actions enough. You don't want to leave your readers confused, so make sure that complex points are explained as clearly as possible.
As a rule of thumb, make sure that something happens in each chapter that is important towards the final verdict of the story. This way the story is constantly moving along, and the pace can never be too slow. Cramming everything into one chapter will do you no favours, so spread out anything important over a few chapters.