A rule to use no formal expressions in certain settings and for certain periods of time, for example: at work between colleagues for a month, in a small town between all its residents, in school between teachers and pupils, etc.
Formal expressions create the feeling of distancing from one another and though one might argue that it is a form of showing respect to the other person, we simply get used to using them on a daily basis with the majority of people except for the circle of our friends and family. This contributes to the feeling of alienation from the rest of society.
Try addressing a stranger like you'd address a friend and you'll see the difference. This is an idea for a social experiment to practically prove this point - use the informal language in settings where you are "supposed" to use the formal one for some time and see if you feel closer to others, more like being in a community.
Different languages have different ways to express formality. Pronouns are usually very important when it comes to this, English doesn't have the distinction between you (as you'd say to a friend) and You (as you'd say to a stranger or an older person) but many other languages do. Formal vs informal greetings are also an important basic part.
But the easiest way to go about this would be to inform people to address others in a way they would address friends or people otherwise equal to them, because to not use "this and that" expression or to use "this instead of that" is confusing.
Someone initiates the "experiment" and informs others to participate - the boss at work, the town office, teachers at school, etc. The start date and time for which this will last is announced. Everyone should participate. Of course, you can't force people to do it, but when someone starts using formal expressions, the others should remind them that this is a "no formal communication week, month, etc." and you should call me you instead of Mr. Stevens.