My first thought is redundancy. You shouldn't send just one copy of the message, you should send thousands and through different methods. Some thoughts on possible methods:
- Rock carving in a protective sheath (e.g. amber or a similar substance).
- Shoot rockets to the Moon and Mars (vacuum doesn't decay things the way that atmospheres do).
- Build satellites in orbit.
- Bury on the sea floor and in swamps (hey, it works for dinosaur bones).
- Hang them in houses for anthropologists to find later.
- Scatter them around the active volcanoes in Hawaii (think about Pompeii-style preservation).
- Impress upon your children that the message needs to be preserved verbatim and have them make copies. Deliberately start a tradition of each generation making verbatim copies.
Again, let me say that the most important part is redundancy. Any single message is vulnerable to destruction for any method of transmission. Make as many copies as you can. That way you have a better chance that at least one will survive.
