Your chakras are always open. To develop them the fastest and safest way, and the most permanent way- is typically through service. If you want to be more clairvoyant, do a lot of healing work and try and look at the body more.

It really depends on what you are trying to do and how you are trying to do it, and I guess WHY you want to is another question you might want to ask yourself. Your chakras hold ancient seeds, and the more activated they become, the more ancient stuff starts to leak out... just make sure you are truly ready for what you are asking for.

Imagining will only temporarily open them, just like 20 push ups will only temporarily harden your muscles. You need to work the energy over a long period of time before it is highly activated and energized. But you also want to open them all in proportion to one another... or you will become imbalanced, or your life will become imbalanced.