In The Relaxation Response, Herbert Benson and Miriam Z. Klipper site studies concerning meditations with and without special, super-secret mantras. If I recall correctly, they found that whatever words, sounds, images, or thoughts were particularly meaningful to the individual tended to produce as good as a response as special mantras.
They emphasized this finding because in TM the instructors say that saying the "wrong" mantra might be dangerous or what not. Or that just saying a normal, everyday word or phrase isn't as good as the mantra they give you.
If you don't necessarily feel the need to have the words you repeat linked to an esoteric, mystical tradition then you can simply search your inner self for the words, images, thoughts, or memories you wish to let your awareness rest upon as you meditate.
