I think the only way we can hope to colonize distant worlds is to have frozen embryos drop into some kind of a womb about 20 years out from the planet we have identified in a distant solar system and have kids become adults by the time they get to the planet. There is no way we can keep people alive across that much space.
Or, alternatively we could try not screwing up this place and then maybe we wouldn't have to go elsewhere. Call me a treehugger, but I think that might be the best solution.