The key thing that has worked for me is not to concede points, but to effectively offer a concession to your opponent in the form of a question. In this way, you draw them as much as halfway toward your viewpoint. (If you are truly good, you may actually see some of their point and move in their direction as well.)
In arguing with a fanatic, sometimes one must be artful with the truth. The best concession is one that completely neutralizes your opponent's point.
Opponent: "String theorists believe their theory solves every problem known to man? How about (some point that string theory may not answer)? They're pulling that outta their asses!"
Responses could include (if available), a factual address to the claim in the form of a correction, an admission that they don't have all the answers, or:
Me: "String theory doesn't have all the answers. It's a pretty well-supported model in that it explains a lot of phenomena in the universe, but of course as with any side, the loudest people will claim to know everything. The truth is, they don't, but if they admit it, then the people that DO claim to know everything will rip them to shreds!"
This has happened a number of times (although never about string theory and always about something much more controversial). It didn't always take this form, but nevertheless it did always demonstrate how keeping a perspective wider than the scope of your opponent's arguments often helps in dealing with fanatics.
Don't allow yourself to be swung about by other people's arguments on all matters. It is one thing to be a student, but it is quite another to be completely without passion and conviction.