we usually talk about each of our needs, and do what will best meet that need.

Had a crappy day at work, Kids driving me nuts, Want subspace to float in for a while.

or

I feel magnificently powerful right now. On your knees, bitch!

That old saw... communication communication communication

Coin toss equals random chance, so I'm assuming you want something other than random chance.

So what about playing a game that you are both equally good at? The victorious winner gets to be the Top, the vanquished loser is the bottom. (This is assuming that you can find a short game so that it doesn't consume the whole evening leaving no time for the play).

Another way would to be for Partner A to put out some symbol of submission where Partner B is sure to see it - perhaps a collar just for sake of the example. This would indicate that Partner A wants to play but is undecided as to which role. When Partner B finds the collar they can either put it around their own neck (meaning they want to bottom) - or they can put it around Partner A's neck (meaning they want to Top)- or they can leave it where it is (meaning they don't wnat to play at this time).
If Partner A is sure they want to bottom, they can put the collar around their own neck as an invitation to Partner B to Top them. Partner B can either accept the invitation or not. If Partner A wants to Top, they can try to put the collar around Partner B's neck. Partner B can either accept the collar and submit, or decline the collar if they don't want to bottom at this time.

Another way would be to write out a number of roles and/or activities (half of them Top, some bottom roles / activities) on small pieces of paper and put them in a jar (I found a jar that says "Fantasies" on the side). If both partners want to play, one reaches into the jar and picks out a slip of paper. You act out the roles / activities as directed by the paper.

Rick

Well, this one has been done mostly in fun and in more social settings, but it had it's very interesting moments.

Understanding there are no set "rules" that everyone follows, another switch and I simply decided that the one on the right was the bottom / submissive.

It is kind of like playing "Simon(e) Says." You are trying to pay attention. If you are feeling toppy you don't want to be on the right. You know what it means if you are on the right. You know that you could be instructed to do something, told you can't do something, have some type of pain inflicted on you, etc. but damn if it just isn't easy to forget sometimes! Then there you are, standing on the right, and the bottom / submissive till the other person makes the same mistake!

However, it also allows for someone to purposely put themselves on the right if they are feeling more in bottom space. If the other remains on the left / top / dominant side, then the silent negotiation is done. (or vice versa)

interesting.
now I know how my friends that are both 'versatile' how they would decide who takes what role.