Wearing the PCs Down

Left to their own devices, a lot of player groups won't move forward when they're down valuable resources. Large spell slots, daily abilities, etc are all valuable and everyone recognizes that their ability to survive a fight without those abilities is much reduced. This leads to players wanting to rest, sometimes even when their characters have only been up for a couple minutes if they get hit with a big fight right off the bat. It can also lead to GMs feeling like they can't really challenge the PCs because good use of big abilities wipes out most encounters, which leaves the GM with setting up challenges that need to be able to stand up to the PCs when they're 100% fresh and good to go.



The Solution
The solution to this problem, and one so obvious that it is presented in the Players Handbook and DMG for 5th edition as how the game is supposed to be played, is to have multiple encounters in a day. This then gives the PCs a strategic element of deciding when they blow those big items, and when they try to hold onto them. The difference between a Warlock and a Wizard is that if given 4 encounters with a short rest between them (the rules recommend 7-10 a day) the Warlock can blow their top level spells in every fight and recover, while the Wizard can only do their top level spell twice - but has more lower level tricks to also throw out in between those big booms.

In other words, you up the challenge for later fights not by making them mechanically harder but by eating into the players resources (hit points, spells, healing items, etc) before the players get to that fight.

The Challenge
The big challenge here is that you can't just force the players into encounter after encounter without it feeling a bit railroady. The whole point of RPGs is that the players are free to choose what they do, so it's not like a computer RPG where you can just deny them getting to the next save point where they refresh their HP/MP and can save the game again. There needs to be a reason that they can't just take a breather whenever they want and refresh all their stuff. Which also makes it less of a challenge, and more of an opportunity.

The Setup
In the console RPG Suikoden there is a part where the protagonists have to save someone from a vampire. As the story in the game goes, the Vampire is going to marry this person at midnight - turning them into a vampire. It's the kind of setup that sets a timer and tension. So imagine my surprise when my friend got that, then traveled halfway across the world to sleep in an inn, buy some items, do some encounters, sleep in another inn, come back, do the adventure and arrive just in time.

Yeah, it definitely breaks the suspension of disbelief.

However, in the computer RPG Quest for Glory II: Trial By Fire, the player is tasked with saving the city from elementals. You can choose to ignore it, but if you ignore it for too long (a few days iirc) the elementals destroy the city and you lose.

You want to set your adventure up to be like Quest for Glory II.

The Execution
While not necessary for every adventure, keep track of time in your game and make it a factor. Give the players a reason to push on after a fight. Did they wake up, run into a couple trapped doors, and get into a fight now they're feeling a bit worn down from damage? Why would they move forward like that? What would compel them to?

Reasons like stopping a ritual set to happen at midnight, saving someone who will be killed at dawn, returning with evidence before a friend is condemned to slavery aboard a ship departing with the tide all work. Less dire, but even things like "the enemy is preparing their defenses, so the longer you take to get up there the more setup they're going to be" can also work.

The point is, give a reason for the players to move and they will move. And if they don't move, let them have the consequences of not moving. Did they take an extra long rest and so the princess is now a vampire? Oh well. That is on them. The first time it happens it will teach them a valuable lesson that this isn't like console RPGs that wait for them to arrive and then pretend it is just on time no matter how fast or slow they went.

A Word on Variance
You don't have to do this all the time. In fact I encourage not doing it. Some days give them 1 fight per day and let them blow everything. Some days give them multiple encounters. Give them easy fights. Give them hard fights. Use variance. Not only will the random reinforcement that being careful with resources work better to keep them on guard, the variance will also make the more challenging fights stand out more because every fight isn't perfectly calibrated to them. The world will also feel more real with some big fights and some little fights outside of the big quest fights.