Personally, I agree with John Wick (I posted the video earlier, but you can find it on Youtube if you search for "Power to the Players") that you really don't need to worry about making your players too powerful too fast. Don't worry about a player twinking out their character, because y'know what? There isn't a character out there that the GM can't make one better than. Y'know why? You don't have to worry about points or whatever when you make the NPC, and you have a wonderful example of how to do it with that character's sheet. What really matters isn't what the character can do, it is what they will or won't do. If you are doing a character driven story, you can challenge any build, then the build doesn't particularly matter now does it? You just cater to what you have to deal with.
So, why, if I believe that am I writing a piece on handing out XP? Well, because there is another matter with XP that doesn't really come up all to often in discussions that I've seen. It's not a question about how slow or quickly your characters achieve their power. It's a question about how consistently they do so, and consistency can be a huge step in helping keep your game feel natural.
Now, generally speaking most games will give a set amount of XP per session. In the Table Tops around here you get 1 for showing up, 1 for goals or objectives your character has achieved, 1 for something your character learned, and 1 for RP discussion. In total you get about 4 XP per session, sometimes 5 or 6 depending on achieving a major objective or learning something huge. Now, this isn't a problem, but what happens when a session is bigger than normal? What about when it is smaller?
Now, by bigger I don't mean how long it takes, but what happens IC. If in general you go through 3-7 days worth of IC story per session, and that is full of learning, experiencing, training, and conflict and that gets you 4 XP. Then some session you cover much longer, or the challenges and conflict are much bigger, still only getting 4 XP may feel like too little. On the other end, the GM has an off day, and the entire session is spent on one day where not all that much happens. Not that it's not fun, but really not all that much happened. Getting the usual XP seems a bit much here now doesn't it?
This really isn't about keeping your players from getting too powerful, or making sure they get powerful enough so much as, like I said, you keep the development consistent. This may be something that only bothers me, but it bothers me as a player when I find myself with the ability to jump a significant amount in ability after a session where my character shouldn't have developed that much. More so then when I don't have enough XP to get what abilities I think the character should have at that point in time.
So, aside from being consistent what am I saying? If XP and your PC's power level is something that you are concerned with, you should spend time before the game planning out about how much you're going to give per session. Even if you aren't worried about it, having some idea of how much you are going to give per session will help you plan out NPCs that will show up later for an appropriate challenge. It will also let you know about when your players will hit certain areas of power. I've seen a few games where GMs were both surprised and annoyed when after a year their characters were at a significantly higher level than they were comfortable with, a problem that could easily have been averted had they looked at how much XP was going out each session and just extrapolated from there for where the power level was going to be in a year.
Though, the best way to use this knowledge of the XP you give and how long until PCs enter certain areas of power is to pace your story. If you are giving out enough XP that people will be entering the higher echelons of power in a year, your story should be paced so you are ready for that in a year. If you aren't doing that, then having your story expecting that level of ability at that time is unfair to the players.
The truly important thing in a game is the Will and Won't of what a character will do, not the what or what not they are capable of. You will always get more fun out of a character by hitting them somewhere other than the mechanics in a game. This doesn't however mean that the reward (and XP in a sense is a reward) shouldn't be considered.
Do not fear putting your players into too much power too fast. However, don't be caught by surprise by it either. Focus on the narrative, focus on the characters, focus on being consistent, and plan mechanics around that and everything will work out.