Personally, I stopped by their offices and took them a little gift (roughly $10 each maybe a little less) and told them the news. I know that is not possible for a lot of people but if the LOR writers are within a reasonable distance then telling them personally is a nice touch. It was also nice for me to be able to tell them. If they are too far to drive to, then a "Thank You" card with a hand written message would be good.