try to practice the five vocal realted to the chacras i,e,o,u,a (from head to coxy) at least one hour everyday

Go check YOga's categories. There are some bija mantra for each of the seven chakras:

Lam, yam, vam.....

About the Mantra OM, if you want to use it to raise your consciousness, pronounce it as OOOOMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM, short O with M prolonged. And to me, AUM is not a substitute of OM, because OM is the Mantra of Agya Chakra(6th) but AUM is the Mantra of Sahasrara Chakra(7th).
    For Aum Ah Hum, there's also another Mantra, that is Aum Hum Ah, pronounced : Aaaaaaaaaaaaaaaaaaaaaum Hum Ahhhhhhhhhhhhhhhhhh(the Ah must be said with great pressure, like you want to shout it(but don't shout or scream), that it makes your body shakes). It is a mantra for gTummo or Candali, a technique to produce heat or even fire from one's body and also known today as a form of Reiki(Reiki gTummo). To practice the Mantra, one must chant it 2.5 hours a day for 21 days, then the gTummo faculty would appear in the practitioner.

I think just using the mantra helps with meditation, you can feel when the sound created is right as the body does vibrate with the mmmmmmmmmm part