Do not use anything that is jarring. In a short time you'll find yourself automatically becoming aware that twenty minutes has passed. Stop the mantra repetition and take a couple of minutes to slowly return to normal awareness. Do not open your eyes or stand up too fast. Allow your body and mind to readjust.
Many people use a mantra.
A mantra is simply a word or phrase that you repeat over and over to yourself. This is usually done silently, but can also be used like a chant. A mantra can be a meaningless sound, a religious word or phrase, or something with personal meaning to you but always keep it positive. Mantras have been shown to have very positive physical as well as mental and emotional benefits. You can meditate on a problem or idea, a thought or feeling, or a piece of information.