R. Kaplan in his commentaries on Sefer Yetzirah describes an interesting unfication meditation involving the letters Shin and Mem. They represent the two outer pillars of Gevurah, Severity, and Chesed, Mercy. As described in R. Kaplan's commentaries, one says these two letters with the vowels o, a, e, i, and u, keeping first the o on Shin, and cycling thru the 5 vowels on the Mem. Then, we change to "a", and cycle again. This is done by breathing once per vowel. Also, this should be done by first contemplating some unification which we desire to have before starting, and by impressing on ourselves the firey image of Shin and the watery image of Mem.