I'm regularly the guy telling people that it doesn't matter what the PCs can do, but rather what they will do. This is a mentality behind GMing that isn't worried about how much XP is given at certain times, or whether or not one character is level 20 and the other is level 5. It's a mentality that thinks character balance isn't something you need, because the GM controls the challenges. However, this isn't the only way to GM, and there is nothing wrong with this. So, today, let's take a look at how you can plan out the power progression in your game.
Why Do This
Before looking at how, let's look at why you would want to do this. Simply put, not every game is meant to be an epic story with people of different wants and power levels. Sometimes the story you want to run needs a set power curve. Or the game itself, the players in it, or the characters being made need a bit more balance for everyone to have fun. Hell, sometimes, you just want to focus more on the game aspect of it and have some fun that way. There's a lot more to it than just this, but these are some basic reasons you may want to do this.
Fast, Slow, or Moderate Progression
The first thing you need to do is decide if you want people to have a fast, slow, or moderate progression. Depending on the game, you may even want a mixture of the three for different arcs. You also want to discuss this with your players. Let them know how fast you are planning on having people progress, especially if it is going to be a slow progression. It can be frustrating as a player to be in a slow progression when you were expecting something faster.
XP Per Level
The second thing to do is look at how much XP it takes to go up a power level. This is easy in games like D&D and Dark Heresy where a set amount of XP gives you access to the next level, but harder in games like L5R where the characters choose how they spend their XP, and ranks come at different rates depending on what they buy. Even so, you can still get a general idea of how much XP is needed to boost characters up to the next level. So, find that value.
XP Per Session
The third, and final step, is to then - based off your planned progression - to decide how much XP you are giving per session. You actually want to keep closer to XP per session rather than XP per monster kill. It makes things a lot more managable and easier to keep track of. It will also help keep everyone on an even growth curve, and make it easy to see who missed what sessions.
Monitor The Growth
Yeah, I know I said the last one was the final step, but you need to keep track of the growth as it goes a long. People running this kind of game often have the problem where growth happens too quickly, and their left not knowing what to do. As I said in the intro, this isn't a huge problem, but if you want growth monitored than you want it monitored. So, well, monitor it.
Control The Challenges
The last bit of advice is to control the challenges. A part of growth is what the characters have done, what they have accomplished, not just how much XP they get. It's kind of hard to expect a slow progression, when in the second session the PCs save the world from ending. Or if on the second adventure they're going off and taking out entire PMCs on their own.
This, more than XP, is I think where most GMs lose control of player growth. Especially when not doing XP per session. The PCs pull off ballsy things, and you want to reward them. The next thing you know, you have level 10 PCs when you were expecting them to be level 5. So, try to keep it under wraps if you want this kind of game.
In closing, this isn't the kind of game I generally want to run, but it can be fun if done right. Just remember, the best way to handle it is to plan out the important stuff, and keep track of them as things go along. Don't be afraid if you lose track of the plan, but try to hold onto it as you can. Most importantly, have fun.