I just found a site listing all sorts of Mahayana sutra translations that are not available in print — including the Nirvana Sutra! It took me over a dozen years of searching to get just a photocopy of the English translation of that sutra produced in Japan. You can find it at:

Nirvana Sutra pdf

The big find was also the Usnisa Vijaya Dharani, which you can recite every day to help the hungry ghosts, your parents, and yourself … preventing everyone from falling into lower realms upon death. A dharani is a ritual speech as opposed to a pure mantra - and for more details you can read  Japanese Shingon (Esoteric School) Master Kukai and his writings.

I won't tell you about the two supernatural things that happened when I chanted this dharani (mantra) for the first time  just to "try it out," so-to-speak, but all I can say is that I'm satisfied as to the truth of the stories. Don't worry about getting the Sanskrit pronounciation right. Just do your best. Try it and see what happens and then if satisfied continue the practice. Read the sutra for the details of its benefit:

The dharani:

 Source: http://www.geocities.com/usnisa_vijaya/web/eng/english.htm

Another Usnisa Dharani story…with this version of the mantra: "Om Amrita Teja Vate Svaha."

Thanks to a reader (that's you Rene), you can hear the Sanskrit audio pronounciation of this at:

http://www.lioncity.net/buddhism/lofiversion/index.php/t9990.html, specifically ftp://ftp.buda.idv.tw/music/ZSZ01.mp3.

Also, an English translation of a sutra about the Zhunti (Cundi / Zhuenti) mantra!

This sutra lists the mantra as

NAMO SAPTANAM SAMYAKSAMBUDDHA KOTINAM. TADYATHA: OM, CALE, CULE, CUNDI SVAHA. 

Because of transliteration (and absense of some Sanskrit sounds in the Chnese language), the Chinese usually pronounce it as:

NAMO SADONAH SAMYA SAMPUDOH  JEETZUNAH DAZATOH: OM, ZHRLEE, ZHULEE, ZHUNTI, SOHA.