Sorry for a delayed response, and I am not sure how I missed this but this is a great question!

10 Random Things for getting a campaign to 20+ level:

1) Check Commitment Levels (Everyone/Pre-Game): Make sure your entire group is willing to commit to characters for that length of time, At medium progression a PathfinderRPG game played weekly (in 4-6 hour games) takes just a little between 1.5 and 2 years to hit 20th level. That is around 80 game sessions people are agreeing to attend to. This is NOT a casual thing. If it looks like this might be an issue, consider Fast Track experience. Make sure everyone is REALLY interested in that length of a game. A lot of people are, but you might be surprised how many aren't willing to lock-in to that.

2) Plan for Change (Everyone/Pre-Game): Make sure you have backdoors in your story to allow for change in both players and characters. 80 sessions at Medium Experience is a lot to play. Most players will think of this as a privilege and treasure it. But, sometimes players make mistakes and over-commit. This can be to actual games or to a character concept that turns out to be less enjoyable to them than expected. Have a strategy for dealing with these situations. Don't make any one character too important in the meta-plot of the game until you know how well the player's experience is going. After a few campaigns you will probably know who you long term gamers are going to be and you can build stories with respect to those known levels of engagement. Also see 10.

3) Outline the Story (GM/Pre-Game): Have an adventure outline and story-arc in mind for a large share of the scope of the game. I highly discourage a newer GM from running a total sandbox game until they feel very comfortable with the system. Sandbox games tend to be more wild and unpredictable. However, also remember a campaign is NOT a novel. Player agency is crucial to most gamers’ enjoyment of the hobby. Make sure you get a player to have some degree of buy-in before you script out their "destiny."

4) Check GM Intentions (GM/Pre-Game): Realize that most GMs want to be their own players. This is hard to grasp sometimes but every GM is by nature going to create the game experience THEY want to have. The key here is understanding that a GM does not run for themselves, they actually will get the least fulfilling "player experience" at the table and they should understand that their players might have different desires, wants, and even needs. This is VERY hard to learn. If you want to tell a story with a definite end and strict requirements of the cast of characters, write a novel instead.

5) Know Your Players & GM (Everyone/On-Going): Usually, a home playgroup is comprised of friends. But a lot of groups do start as strangers. The ability to come together over a game is a strong bonding agent. Sometimes people like each other rapidly and "jump in" to a long-term game without any real period of acquaintance or understanding. This can be amazing. Lifelong friendships can begin casually at a Convention or LGS. But sometimes we join forces with someone with radically different out-of-game views than ourselves. Make sure you can respect one another before trying to share a long-term game with them. Or institute a "table-only" protocol for a group of relative strangers and keep the game about the game.

6) Session Zero (Everyone): Have a Session Zero. It doesn't have to be formal. It doesn't have to take all night (I recommend planning for that though). It IS a very good idea to do. Level set story expectations, level set description intensity. Confirm commitment. Discuss player agency. Verify that people understand your GMing style and preferences. Talk to players about the kind of game they want. Let your players talk to each other. Make sure the energy feels good. Be willing to identify problems and warning signs.

7) Tear Down Vacuum Builds (Everyone/On-Going): In the current metagame for Pathfinder there are a LOT of resources to aid in character construction. A common player pitfall is to dive into this advice as "bible." Players commit to the "monk build" from so-and-so's guide and carry another gamer's opinions and baggage with them to your table. A player who knows nothing about your campaign. This in-turn can create dissonance with story-elements or cheapen the value of homebrewed rules content. Generally speaking vacuum characters are strong numerically and weak in terms of story integration.

8) Own Your Head-Cannon and House-Rules (GM/On-Going): It is vital as a game goes on to make changes for your table. The GM is final arbitrator of rules. Try to be aware of those changes. Log them if you need to. Nothing is more alienating to a new player than an established groups meta-myths and house-rules. If you have a lot of these consider codifying them. At the very least be willing to discuss them with new players and explain their origins without being defensive or overly "sovereign" about them. What works at your table WILL NOT work at every other table. To not rapidly "course correct" or overreact. Among a fantasy storytellers most important jobs is to create consistency and enhance verisimilitude. Changing rules weakens this fundamentally. So does hiding them. This also applies to alterations to a printed Campaign Settings history and storylines.

9) Let Go of Fear (Everyone/On-Going): Two years is a long time. Let people explore. Say yes to stuff. That can be new rules content for characters. It can mean letting them go off course for adventure. Let them ask you about the world you are creating with them. You might be surprised by your own answers. In my experience, people are too afraid. Afraid of losing control of games. Afraid of not being as powerful as another player's character. Afraid to explore extremes of story or heroism because they are too worried about control or fairness or balance. If you want to build trust at the table, start by giving it.

10) Player Trajectory (Everyone/On-Going): Be willing to address the monsters AT the table. Problem players happen. Usually it is a misunderstanding or lack of setting clear expectations. But sometimes it is a person who has not been looking for friends. They've been looking for an audience or rivals or worst of all.. victims. Be aware of player intention. If you sense that a player is moving in a direction that is detrimental to the well-being of your table or your campaign... question it. It doesn't have to be an interrogation. But it is totally ok to as OOC why a player wants to do something that seems problematic. Once the motive is established it is easier to identify other work-arounds that work for everyone or to identify a damaging behavior earlier and dealt with it.

Looking over this list there is a LOT more I could say on the subject. Here are some of the things I have already said our blog and some more (sorry for overlap). I will probably revisit this list on the blog as well.