There are a big number of programming languages. Some of them grow up and become very popular. As people use such language more and more it become very popular. Founder of such language(community) try to bring changes to make it better. But sometimes it's hard to make some changes because of back-comparability and such ugly things exists in languages for years.

Is there any architectural principles, steps, during language design which can help to make it more stable and don't afraid to brake back-comparability?