1. Check the furniture first — thirty seconds, before anything else
Before anyone sits on anyone, check what you'll sit on: no wheels, no wobble, nothing that slides on the floor, and rated for two people moving on it. Height matters too — the straddling partner's knees should bend at roughly a comfortable level, not fold into a deep squat.
Check the exit path: both of you should be able to get off without stepping blind or twisting. If nothing stable is available, the floor with a cushion against a wall is the same idea — a firm surface and something to lean on.