try putting an ice pack on your hand for several minutes (to get the skin to shrink back a bit especially useful over the knuckle) Then put oil like olive oil or even a heavier oil like crisco all over your finger around and underneath the ring if you can and rather than try to pull it off - sort of twist back and forth quickly as you move it down... keep repeating until it comes off.
When I used to sell jewelry inevitably someone would put a ring on that was too small and get it stuck and that was the trick we used. Lotion can be used in place of oil too in a pinch. This trick works quite well. good luck! :)