I realize that this may be one of the intentions of the US Electoral College system. Its most obvious role is to prevent a candidate of purely local appeal from winning nationally, but another de-facto role seems to have been assumed by many media sources in the recent election. Specifically, I got the impression that the EC members were supposed to exercise some sort of double-check on the candidate's validity, character and legal qualifications (e.g. lack of conflicts-of-interest) and thereby moderate the influence of pure populism on the outcome, at least to some extent. A few of the electors seem to have interpreted their duty in this way (Christopher Suprun being the most public example).
If the latter truly was within the intention of the Founding Fathers, it means that it's not ridiculous to consider implementing a brake on pure populism—without meaning that one is cynically discarding the notion of democracy entirely.
My question is: what other electoral mechanisms might achieve this?
Digest: answers seem to cluster around several ideas:
There's nothing that can be done about demagoguery if you want democracy. I would say this is neither helpful nor entirely true. The goal would be to make the rise of the next Hitler harder and less likely, while acknowledging that it can never be ruled out entirely. Furthermore, there are many shapes democracy can take and mechanisms could always conceivably be improved and adapted to current conditions (similarly, sentiments that the US Constitution is already perfect, or inviolable, or even just the best imperfect system there could ever be, are all highly debatable and at best lacking in the vision that the spirit of the question hopes to elicit).
Educate the masses. Always a worthy goal in general, but if one were to try to do this systemically, decisions about the syllabus content would disappear down a million subjective rabbit-holes. There might be ways of crowd-sourcing it democratically, but its application would almost inevitably be uneven. This answer by Philipp to another question gives a great run-down of all the mechanistic problems with applying this truly democratically (especially if considering making the franchise contingent on it).
A panel of experts to vet candidates, before or after election. This could easily be done badly (i.e. in a way that is so open to abuse that it would only reinforce tyranny). It might or might not conceivably be done better than currently so is worth debating.
Tighten the qualifications required to run. The most objective way to do this is to require legislative experience, though it's not clear that the outcome of that would be any more desirable. Other evaluation methods would run foul of subjectivity to more or less of an extent. To my mind, one interesting idea that emerged is that of applying an electoral handicap (not an all-or-nothing bar) to candidates based on the non-verifiability or disprovability of their public statements.
Smooth the process of becoming head of state with respect to time, in various ways such as indirect election by a rolling elected panel. This might tend to allow popular frenzies to cool off.
Reduce and/or devolve the power of a central government, and/or the head of state, entirely. Great in theory. Practicability thereof is an old and well-worn debate.