There are specific Mantras with specific porposes that to be done in specific time and/or even place. But there is also Ishta Mantra or Individual Mantra that can be done all the time and anywhere. That's the Mantra that can accelerates one's spiritual growth--& also my main practice. Ishta Mantra should be a short one and felt natural for the practitioner. Some of those, for instance are: AUM, OM, Elohim, La illaha il Allah, Om Namo Amitabha, Isis, Nuk Pu Nuk, Dana, Jesus Christ, and many others. Pick a Mantra that close to your heart, then chant it as often as possible, anywhere you are.

i agree. i am still searching for a guru.

i have been chanting mantras for several years, over the years i have chosen 5-6 mantras that i feel an affinity to.

there are 2 occasions when i woke up chanting "Om Mani Padme Hum"cos i have a bad dream. i wonder if this is the sign that the mantra is working.

i also sing out the mantra if i feel very stressed and down.

This is my experience, but it's not a postulate. Anyone must try and discover for their own what is right or wrong. Based on what I had experienced, it's not good to use more than one Mantra at once. I did that once, and it gave me trouble, both physically and mentaly. If I want to use another Mantra other than the one I'm using, I must completely stop my chanting practice for at least 1 day before using that new one. I think it because each mantra activating different energy point(Chakra), so mixing them would mix also the energies inside my body.
    And yes, every Mantra containing Bija(seed) within it. It is something that planted in our subconscious mind. That's why anyone who ever practiced any kind of "Mantra Yoga" but then stop practicing for a very long time, can still have his/her Mantra pops out in time of stress/pressure.
    Also if I may give a suggestion, you can try practice your chanting while you lie in your bed waiting to fall asleep. It's a good practice, because--maybe you already had noticed too, the last thought that you have in your mind before you fall asleep, determine the dream that you will get.

MacLir, your wisdom also fits my thirty-five years of experience with things.

love,

sunmystic

Not really into mantras and such stuff then until events made it happen that I get to meet a local guru (he's in heaven now - may his soul rest in peace).  My experience yet on this is limited but it is indeed very helpful.  When I got to handle a really serious case that some (and even I) opine is caused by local witchcraft but what my guru friend opined is just an influence of the alignment of stars then the Maha Mrityunjaya mantra really helped a lot.  Although it took quite a while (a year to be exact) for the subject to have a normal life again as she really had it bad the others and even I who used it experienced positive changes after just a short time of using it.  We just play it in the background while we do our prayers/meditations.