I've run and played a couple 5E campaigns now, and one thing that bothers me a bit (and this bothered me in 3E too) is how quickly players advance through the levels relative to the passage of time within the campaign world. At standard number of encounters per day or per adventure, and using the guide for how adventured are packed together the way the hardcover campaigns pack them in, you can advance from Level 1 to Level 20 inside an in-campaign year.
If you think about, that's crazy! Not only is that now how real life works, that's not even how any of the fiction that D&D is based on works. Aragorn wandered the Wild for decades. Conan became a King as an old man. Paksenarrion was on campaign for years. The companions that met at the Inn of the Last Home had been separated for five years.
Adventures should be spaced out. The timing is yearly or seasonal, not penciled into a day calendar.
So I want to share what I have done recently, and I think is working quite well:
1) PCs do not advance in level during an adventure. The level you start the adventure in is the level you are for that adventure. (The exception is Levels 1-3. Arguably, 5E characters aren't fully themselves at Level 1, so you can advance up to Level 3 within an adventure).
2) Extended periods of downtime between adventures/levels. The default assumption is that you have a role in the campaign world (usually related to your Background), and that's what you spend most of the time doing. There aren't professional adventurers who are adventuring all the time. Adventures are something you go on when the opportunity arises. Like Bilbo's journey to the Lonely Mountain, or Flint Fireforge letting his smithy go cold.
3) Level up two levels between adventures. Adventures start at Levels 1, 5, 7, 9, etc.
4) Give players the flexibility to go on side-quests during the off season, or craft items, or start building a castle or temple or something. Stuff that takes weeks, months, or years. You can get started with that at the Level 3-5 downtime. No need to wait for when the PC is retired and no one cares anymore.
5) Introduce the trope that there's a mechanism why which the gang gets back together. "Evil is afoot. Old allies need your help. Meet me in Palanthus on the 1st of April."
This has a number of benefits.
A) Fictionally, this seasonable timing is just more believable in a lot of ways. It's actually a suspension of disbelief that someone can become a master of their craft in a month.
B) It gives the world time to grow and react to the things the PCs do. When they're level 7 they can return to the castle they cleared of monsters at levels 3 or 5 and see that people have moved in and resettled the area. The songs of their heroic deeds have time to be composed and precede them. Their enemies have time to lay and grow more complex plots to stop them. Etc.
C) No tracking XP! Just don't even worry about it.
D) Opens up new parts of the game, like strongholds or extended research and crafting.
E) Allows players to switch out PCs within the fiction. Tired of that human fighter? I guess she joined the Blue Dragonarmy during the years of separation. But look, a cleric has walked into the Inn and needs our help.
F) Episodic campaigns. This is preference, but if you share this preference for smaller, more episodic adventures rather than grand campaign arcs, this arrangement is perfect.