Take a piece of rubber/plastic, I have used a rubbermaid plastic sheet that is used to put in the bottom of a kitchen sink to protect/cushion dishes that are placed in the sink. Plastic needs to be just the right thickness, cut it to slip down in the hammer channel, right around the firing pin. I used a leather punch to make a hole that will fit around the firing pin. Once this is done, you can dry-fire many, many times, and the hammer will not hit the firing pin so no damage will occur. Of course you need to examine the rubber every so often to make sure that everything is ok.
When you are done dry-firing you will need a small crochet hook to pull the rubber cushion back out of the hammer channel.