Training in concentration has had thousands of pages dedicated to it, and there are probably thousands of concentration exercises. Some very commonly used objects of meditation are the breath (my personal favorite), our posture, a mantra or koan, a colored disk, an image, a candle flame, various visualized objects from simple to complex, feelings such as compassion, and even the experience of concentration itself. The object you choose should be one on which you would be happy to steady your mind.
