From the 10,000 word post:
"In D&D, we not only have curiousity, we have a guarantee that we WILL know, once the right actions have been taken. This is what is promised by a book or a film, too - but in D&D we are not passive. We are active, we make the decisions about what we want to know, and what we'll risk to know it."
To keep the momentum going, you have a powerful ally in the curiousity your players possess. So long as you always keep one more thing on the horizon that is not fully explained, you have the potential for stirring your players to investigate that one thing. People are obsessed with wrapping up loose ends. People always feel, somehow, that it is possible to finally know everything ... and they will foolishly risk all if there's one nagging problem left unsolved. You're talent as a DM depends on your ability to exploit human nature.
If you wish to really give the party an opportunity to direct their own lives, and move through your campaign as they wish without feeling railroaded, then you must provide more than one thing that is unexplained. As the paragraph that led off this post says, what's marvelous about D&D (and what sucks regarding life) is that there IS an explanation. A resolution IS possible. With that knowledge, the party will be spurred to go find out (thus ensuring your campaign's continued momentum).
With more than one puzzle remaining, you will find your party debating vigorously on which one to pursue. This is more than merely having two doors available for the party to choose from. A door with something behind it is mildly intriguing. Elaborate puzzles, however, require a greater investment than turning a door knob.
What is an elaborate puzzle?
For too long D&D has been sold upon the idea that a complex puzzle is some physical thing with a lot of moving parts, which needs a trick to solve. Pull lever A, then release the water from bucket B, catching it in sluice C and directing it into reservoir D, to which must be drained into pipe E and flushed through spinwheel F ... and so on. This is not the type of puzzle of which I am speaking.
The greater question is not how the above puzzle is solved, but why the above puzzle exists at all! Why would someone build it? What purpose does it serve? Surely, it is possible it has a greater ideal behind it than just to annoy party members. What if, in addition to the contraption itself, there is a signature affixed to the bottom? That signature would immediately become the most interesting thing about the whole mess - particularly if that signature promised the possibility of meeting a real person, who in turn enabled the party to grasp some greater significance to the construction of more elaborate puzzles.
The puzzle itself would be forgotten in a few hours - but the signature on the puzzle would baffle and intrigue a party for weeks. Finding more signatures, and more information about the creator, and clues to where the creator might be, and how the creator's influence is affecting all the people of a given realm - that is a campaign that has the potential to drive the game for months.
If the party solves the puzzle on their way to defeating some monster, upon defeating the monster there is sure to be someone who says, "We should find out something about the man who signed that annoying puzzle." If you as DM then have some ready information to give the party in answer to that query, your campaign will not have to wait to regain momentum - the party will be off and running again, and they will do it believing that they are mastering their own destiny. It was their idea to investigate the signature, no?
Fill your campaigns with little clues that have NOTHING to do with the present adventure. A caravan crosses the party's path with no purpose other than to bring a dress to the local princess who is marrying (against her will, according to the master of the caravan) the local prince. Seven dwarves are digging with shovels in the middle of a stream and seem really unwilling to talk about why. A young girl stumbles out of a nearby forest, is unable to talk, and kills herself with a knife the first chance she gets to be alone. Different people along a road seem unable to stifle their laughter whenever they see the party's assassin. A shopkeeper refuses to sell the white-booted horse to the fighter, although it seems no different from any other horse, and the shopkeeper denys that he has a buyer - he just won't sell it to YOU. A look of fear crosses the features of a town guard upon seeing the mage, but he quickly covers it up and now he won't talk about it. A teamster is casually cleaning blood off his buckboard as the party moves past. The river the party means to ford has a very definite blue tinge.
These are puzzles. Any could be ignored by the party right now. Later on, when the party has nothing going, they might debate among themselves about investigating the forest the girl came from, or the blue river, or if the wedding took place. They might spend half an hour kicking it among themselves ... always having it in the back of their minds that if they're curious, they can find out why. You as DM can promise that.
This keeps the game ticking along. You don't have to push your party to go here or there ... just provide a little whiff as they're going by, and they'll come back to the thing all on their on. It depends on how well you sell it. Dungeon mastering is like arranging a confidence game; you have to hook your players in with something that, A) doesn't make sense; B) seems to be below everyone else's radar, so that your players feel smart about noticing it; and C) promises wealth and power aplenty. If you actually provide wealth and power, your players won't feel suckered - or they may tolerate being suckered if it works out all right in the end.
Either way, your campaign keeps moving forward, no one is bored, and half the work of what to do next is carried by the party. You might even get a chance to eat a sandwich while they're arguing among themselves about what to do next.