How do you mark spiritual milestones in your life?

We have been studying Joshua in Sunday School and several times the Bible talks about him using rocks as spiritual reminders. For example, after he led the Israelites across the Jordan, they constructed a monument made out of 12 rocks (one for each tribe of Israel) to remind them of how God brought them into the Promised Land. I think this would be a neat thing to have and to use to teach my children about God and his goodness to us. Do you have any visual reminders of spiritual events in your life? What are they?