Freeform games are fun with a good group of people. Everything depends on an often-unstated social contract, where everyone joins together in the spirit of cooperative storytelling.
Something that I often find happens in regular freeform games is that people don't always get involved very much. So I made up The One Rule:
You should accept anything another player says about your character in /some/ fashion. (unless it really pisses you off, in which case, negotiate with the person who said it OOC, and come to a solution that can include accepting, modifying or redacting it)
Here's the theoretical bit that goes with it, as helpfully articulated by Elizabeth: "If the only character history a player is responsible for is someone else's, it forces the characters to interact, and rewards that interaction through social contract."
That was probably an unnecessary example, but it was fun to do, so you're stuck with it.
Other than that, just work together to come up with a basic setting, and maybe 2 or three lines to describe your character. Anyone can set scenes, play/control NP Cs, and do anything else that a GM would normally do. Just don't be an asshole in an OOC sense.
A page on One Rule Theory
Comments: (to edit: sp4m4l0t )