I found this useful Buddhist Mantra in Lillian Too's book Chant a Mantra, Read a Sutra. Of course the best way would be to get your Mala Beads blessed by a Lama but this method has been shared so people can do this themselves if they wish.
While holding your Mala in your hand chant the following mantra seven times with the clear intention to bless them:
"Om Ruchira Mani Pravartaya Hum"
(Then blow onto your Mala...)
Those interested in learning more about Tibetan Buddhist mantras may wish to check out Lillian's book Mantras and Mudras. The book I used is only available free on request while purchasing Feng Shui items at some websites and stores that stock items from World of Feng Shui.
I make 'Mala inspired' jewellery from both Buddhist and Hindu traditions, some of my designs use specialist Mala beads which have been blessed. You can find out more at my shop Merkaba Warrior on etsy, I ship worldwide.