How can you remove an extremely tight-fitting ring from your finger after 30 years without damaging either the ring or your finger?

Probably the cheapest, most effective method would be to wash your hands in running water just as cold as you can possibly stand. Make sure to use plenty of soap, in particular on the ring finger. As your fingers get colder, they will begin to constrict. Likewise, by working the soap (liquid, non-foaming would be best) on and around your ring, you'll be lubricating the ring. At this point, with minimal effort, you should be able to pull the ring past your knuckle, and it should be a fairly simple process from that point.

Another method is to slide a string under the ring. Then wrap it as tightly as you can around the finger headed toward the end and over the knuckle. By pulling the string under the finger it will slowly move the ring up the finger and over the knuckle. Be careful!

You may want to consider going to a jeweler's and having them cut it off as a safer way of removal.