Like it or not, the Libertarian and Constitution parties are considered "fringe" and come with baggage. An affiliation with those parties will turn off many voters. From my understanding, the LP is known to be too rigid and ideallistic. The CP (which I have on a few occassionals considered joining) invokes Christianity in its platform. I'm a Christian, but I already think the Constitution protects me and my faith enough.
Right now, I think the plan should be to fight like mad to take back the GOP. Things are actually setting up well for this. McCain is polarizing and conservatives are not at all pleased with him. Our roadblocks are currently Paul's foreign policy platform. Too many conservatives are convinced that leaving the Middle East will not reduce the threat of terrorism. I don't think any amount of education or debate is going to change their minds. I was one of them and the only way my mind was changed was by examining Dr. Paul's platform myself.
Simply put, we must get involved in our local Republican party and become precinct chairmen and the like. That way we have a voice and in the process purge out the established leaders who perpetrate the lies and decption we hear about all across the country at the local level.