It takes more than visualising to open your chakras. Visualisation is an important component, but really you need to assess your whole way of life, thinking, and emotional health. They are a reflection, as well as a cause, of how you live.
There is little point in visualising if you are living in a way that is damaging you. If you focus on living as well as you can, your chakras will respond to that. Then, visualising will be like the 'final touch'. It's a bit of a cause and effect thing. The chakras affect how you approach life, and how you live affects your chakras.
So, try to keep a balance with your external and your internal life, and you will feel a difference. Above all, be patient. It won't happen over night! :)