How do I prevent defection

Several ways. You can build some influence starbases nearby to counteract the enemy influence, or build some structures on the planet that boost influence like the cultural exchange center. If you have access to the re-education center, building that on a planet will keep it from flipping.

Tennekis, you don't give any specifics. If we can determine how you got into this situation, then we could give suggestions about how to keep it from happening in the first place. The most common cause is low morale. Keep your people happy, and they will be less likely to rebel. Everything that DethAdder said will certainly help after the fact, but it is much easier to stop the rot before it begins.

Agreed! Over population, high taxes, and foreign influence are all factors on if a colony will culture flip. Its much easier to keep a planet happy then it is to try and "re-convert" them.

If its high population, try building some moral structures... Others have some more through break downs but a good rule of thumb is one moral building for every farm.

High taxes? Thats easy... Lower taxes!

Foreign troubles, usually moral star bases and a combination of the previous two solutions will work just fine.

Additionally I believe it was stated once that having ships in orbit of a planet helps a planet from flipping as well... Can someone back me up on that?