As one of my previous teachers have said, "Even after almost thirty years of working on centering myself, I still have to remind myself. It's kind of like telling myself, 'Here girl! Here girl. Heel. Good girl!'"
One exercise that she gave was to work internally and consciously through embodying the quality that you wish. Every year, she chose for herself a question that she worked with for the year. For example, your question might be something along the lines of, "If I were more rooted (or centered, or calm, or controlled), what would that feel like in my body?" This allows the body to react to go back to the state in which the quality exists -- rather than just saying to yourself in your mind, "I have to be centered! I have to be rooted!" I think this helps people who are especially kinesthetic as it attaches a bodily sense to whatever quality you wish to have.
Centering takes years and years, if not decades, of practice for some. The process of "recentering" yourself mentally/physicall/emotionally (as in "coming back to yourself") may only take five seconds, but you may have to do so a hundred times a day. But, hopefully over time if you work on it consciously and diligently, that hundred will become ninety-nine, ninety-eight, and so on...
Hope that helps.