I am going to derive on the things already said and add few comments:
1) The message must be interesting to pass on Take a story about Atlantis. Somewhere I heard that it actually describes a story which happened 1000 years before Plato. In other words, story old about 3500 years is still around (and people are still having arguments what and where Atlantis might be)
2) Make it into religion: This point will help you a lot to make your message to pass on in generations. Not only people will be willing and wanting to make copies of your message, but also, your message will be translated to many languages (especially if your religion becomes popular).
And also, you can make sure that your language will be still around even when no one actually speaks it.
3) Build whole society on the message Ok, we know about religious practices old about 7000 years, and still are somehow able to understand the language. But how did we learn about it in the first place? Well, they built the pyramids. They built the Stonehenge. They kept their awesome badass burial places behind.
So, to wrap up:
- Make your message into religion
- make the religion very popular and mighty
- Make sure, the culture and society lives on the religion.
But one thing is for sure: Using those practices above will be able to pass the idea of the message, not message itself. Religions update as society updates, and wonders of the modern world get forgotten in the desert, or the fields, leaving no one behind to tell the stories.
Also, war happens so even following all the rules might get you to dead end.
So, if you need the exact message to be around in 50 000 years, use means as stated in other answers