I understand that the best thing to do is to hold onto a good company forever. Let's say I need the money out in a decade's time and the company has grown, how do I monetize that paper gain without exiting the position?

Hello there! When you make an investment, I believe there should an investment objective in mind. One of the possible investment objectives could be to make down payment of a house where you invest in x company so that you can have y return in 5-10 years. When your investment objective has been met, I would say it could be time for you to sell. To be very honest, it is really difficult to know for sure one is a "good company" given that things might be different 10, 20 years down the road.

Hello! Really interesting question. I think the easiest thing to do will be sell small chunks of the company slowly. That will, you get to still participate in the company's growth for a significant period of time.