At the time of writing, it looks like Clinton will end up with 232 pledged electoral votes, and Trump with 306. So 37 electors would have to flip just for a tie.
My understanding is that some states have laws binding electors with either fines, criminal penalties, or the right to replace their votes if they don't vote in line with their constituents.
Assume Clinton's best-case-scenario: every elector would vote for her if their state's law permitted it. Are enough electors able to switch to change the outcome of the election?