culture - How do Japanese speakers transition from polite to plain form amongst friends?

When you first meet somebody, polite language is, on the whole, the most commonly used form of Japanese. Children may differ, and I understand that not everyone will use plain form, but regardless a majority of people do.

At some point in a friendship, inevitably friends will switch to plain form. Again, different people are comfortable to do this at different times, but how do people make the transition from polite to plain language? Do they just start using it, and the other person just plays along? Is it a gradual thing, where you switch in and out of plain form and then just stick to it after a while after using it more and more?

Personal experiences and examples are welcomed