One category of DMing advice that's always struck me as particularly obnoxious is that of how to remove surplus wealth from the player characters' possession. We're advised to levy taxes, send thieves after them, impose hefty training fees to level up, and an assortment of other schemes with the express purpose of keeping a tight rein on the PCs' wealth.
Maybe this is partly an acknowledgment of the absurdity of characters accumulating personal hoards that make a king's ransom look like pocket change. It might be intended to keep the PCs poor and hungry, so they have motivation to keep adventuring. To me, it indicates that there's something fundamentally out of whack in the game. If you even have to ask the question, "How can I take it away from them?" something's not right, and frankly, you're asking the wrong question. Let's try a couple more productive lines of inquiry.
Am I handing out too much loot?
Two reasons come to mind why the referee feels compelled to be overly generous with treasure, one game mechanical and the other dramatic.
The game mechanical reason is that characters need XP to advance in level, and XP is largely a function of treasure recovered. This is where the game is broken, and rather than fix it, the designers offer us awkward Rule Zero kludges. A ratio of 1 XP to 1 gp works fine if all you want is to keep score in an abstract, video game sort of way, but in a more serious game it kills any sense of balance and verisimilitude. If you stick to this ratio, and the by-the-book advancement charts, each character is going to have to accumulate thousands of gp per level. The easiest solution here is simply to alter the ratio - reward more XPs per gp of treasure value, maybe 5 or 10 XP per gp, and reduce the size of the troves by the same factor.
The dramatic reason for treasure inflation is that players expect fabulous treasures,
commensurate with the monsters and other obstacles they have to overcome
to get them. When you take on an ogre and manage to finish him off
before he clubs you into paste, you want something awesome for your
trouble. Books and movies only reinforce the trope of vast piles of
gold coins, sprinkled liberally with sparkling jewels. The answer to
this is simply to ignore the trope, turn on the imagination, stop trying to
impress the players with quantity, and aim for quality instead. A few
interesting items can be as rewarding or more so than a pile of gold,
without all the inflation. Sure, it's a little more work on the front end, but it saves you the hassle of trying to sop up surplus loot later.
What is there for PCs to spend their money on?
So we come to the second problem, an appalling lack of cool or necessary stuff for PCs to buy with their money. Part of the problem is that, in the rules as written, most characters can afford the best non-magical equipment in the game before they earn their first coin from an adventure. A reassessment of the standard starting funds and the costs of adventuring gear are clearly in order, but that's beyond the scope of the present post. (The Jovial Priest takes a very worthy stab at it here.)
Other than consumables like oil and rations, mounts, and perhaps a one-time upgrade from chain mail to plate for the warrior and cleric-type characters who rolled poorly on their starting gold, there's just not a lot that low and mid level adventurers need to buy after character creation. Eventually, they may want ships or even strongholds, but that's still way off in the uncertain future. For the next several levels they're just accumulating loot by the ton. Virtually every "patch" for this problem that I've seen is either agency-destroying, embarrassingly transparent, or both, but many of them can be salvaged. The key is that you entice them to spend money on perceived benefits, not just impose costs on them.
Cost of living: This makes a lot of sense, but generally it automatically rises with a character's level. Why? Ostensibly because higher level characters want to live large and luxuriously. But what if the player conceives of his character as a quiet recluse with simple tastes, or a miser, or a forward-thinking type hoping to be the first of his peers to afford a ship? Instead of just imposing escalating costs of living, make up a chart of various standards of living and their monthly costs, and allow any character to choose any standard of living he or she can afford. Then adjust the character's reputation accordingly. Being a grubby hermit who sleeps in a tent in the woods and forages for roots and berries is cheap, but it won't get you far in polite society. Many players will be willing to bear the cost of a better life for their characters, especially if it opens certain doors for them socially.
Carousing: An interesting concept, proposing that characters don't gain XP for treasure gained, but only for treasure spent, thus encouraging them to blow their cash on wine, women, and song. The problem is that it robs players of agency, punishing those who want their characters to be of a more temperate demeanor. It's fine if it's a choice, with some sort of benefit to a character's reputation, but I don't like it as a condition for advancement.
Class training: When I see level-up training mentioned, it's almost invariably for the express purpose of putting the hurt on characters' finances. Otherwise, there's really no mechanical significance to it at all, and it might as well just be hand-waved away. Gaining levels is a vital part of a character's development, not an optional perk, and requiring payment for it seems like an incredibly transparent ploy.
Weapon/skill training: This actually makes sense, if you use a weapon mastery and/or skill system. Characters pay for benefits that are not essential to their classes, and thus truly voluntary.
Taxes and tithes: Points for realism here. It's hard to imagine a ruler who would let filthy rich adventurers off without contributing to the royal coffers, but again, automatically deducting the amount from the PC's account is a violation of agency. If a character wants to evade taxes, let him try. He may get caught if he's sloppy, and even if he's not it may come back to bite him later, when the king wonders how he has the funds to build his own castle but never paid more than a copper in taxes the last five years. At least he had a choice in the matter. If he wants to stiff the church, so be it, though the choice may earn him a chilly reception when he needs a curse lifted.
Theft: As a wealth-reduction measure, it's one of the most arbitrarily ham-handed imaginable. As a fact of life modeled within the game, it could make for some interesting choices. I'd recommend never simply declaring loss by theft through DM fiat. Rather, after assessing the PC's reputation and the precautions he takes with his stash of money, I'd suggest a percentage chance per month of an attempted robbery, and a chance of success. This gives the player an opportunity to plan and invest in additional measures to protect his wealth, if he so chooses, and if he does end up a victim of theft, it's through his own choice plus the caprice of the dice, not DM malice.
Specialists: It's certainly legitimate for PCs to pay for services, but I can't help but think that some of the prices are contrived purely for the purpose of separating them from their wealth. 500 gp a month for an animal trainer? Really? A 1-gp per month farmer is probably capable of training a dog or breaking a beast of burden to the yoke. Training a war horse or a falcon probably is a more specialized job, but 500 times more? It strains credibility a bit. If you pare down the amount of treasure you hand out, I'd say it's safe to slash most of the specialist fees by a factor of ten.
I don't feel like I've actually solved all the facets of the problem just yet, but I've taken a shot at identifying them, at least. I might see if I can come up with a more comprehensive plan to adjust the D&D economy to non-absurd proportions - revised starting gold rules, equipment and service lists, etc. Perhaps that should be my first Game Supplement Download project.