All a third party has to do, is be there, in the wings, ready to descend like a vulture, whenever the Dem's or GOP get too ignorant.  That constant possibility will put some checks and balances on them.  And with some actual political hopes, a third party will begin to attract better people.
A down and out person dreams all kind of grandiose plans, but if they somehow come across real opportunity, then their dreams are set aside, for the practicality of their new opportunity.
If a third party suddenly sees that the presidency is an actual possibility, and no longer wishful thinking, then they will begin realistic planning, pushing all their wild dreaming, and their fanatical members aside.  They will buckle down to real business.