I started meditating on the chakras about 2 years ago I think. It was quite early on after I first started to sit and meditate. I can't remember how it exactly came about, I have a very deep interest in the chakras so I think it just happened organically. I do remember thinking that it was perfect because there are 7 main chakras and that I would meditate on a specific chakra every day of the week :) Then the mantras came, one for each of them. There are many that can be used and it really depends on what resonates for each person. Also, as it just happened to me, the mantras may change along the way depending on the circumstances, current situation and the personal growth. I used the same mantras for about 2 years now but, again quite organically, they all slowly shifted to something else, something that resonates much more with me at this moment and time in my life.
Muladhara chakra: I am exactly where I need to be. is now: My path in life is revealed to me.
Swadhisthana chakra: I let go of ideas which no longer serve me. is now: I let myself be carried through life without resistance.
Manipura chakra: I surrender my negative qualities into the fire of transformation. is now: I know my own power and I take responsibility for every part of my life.
Anahata chakra: I am sorry, forgive me, I love you, thank you. is now: I forgive myself and free myself for all thoughts of guilt and pain.
Vishuddha chakra: I am limitless. is now: I am That I am.
Ajna chakra: I am union. is now: I am peace.
Sahasrara chakra: I am one with the universe. is now: I am light.