Trying on rings is always a risky business, especially if you don't know your ring size. Slowly but carefully, you slip on the rings you like and pray that they do not get stuck. There is nothing worse than trying to yank off a ring that is stuck on a finger as it slowly starts to swell. Some different methods and liquids can be used to remove it, but Vaseline might be the best.
In the past, butter was used, but Vaseline is a lot less mess than pouring melted butter on your hand. Dab a little bit around the ring, and it should slip off.