Best way to preserve a stratification system?

Control of ideas and education are probably the most efficient methods. Don't bother using force, because those being forced will probably push back. Make the people afraid to think outside the box, and use media to fully criminalize those who do. Don't let the teachers teach ANYTHING other than basic skills, how good the government is, and how much happier every citizen is compared to those in other countries, especially if that's not true. Convince the people that they have control over their lives, but really, hold the strings of education and opportunity as tightly as possible. That, my friend, will keep you in power for a long, long time.