. . .

After someone has left their seat,
sit in it until their warmth has gone.

.

variation, for a large group in a public space:
ring the perimeter of the space
keep an eye out for people leaving their seats
perform the score above and then return to stand watch again on the perimeter.
don't be super-noticeable in your pursuits,
but do be quick enough to obtain a trace of heat.
however – it's not a race (yield to others)
and if someone "beats you" to your destination,
continue walking on to the other side of the perimeter.

. . .