Consider a party of 4 PCs fighting challenges with CR equal to their class level. According to the principles of CRs, that party should be able to handle about 4 or 5 such encounters per game day. My experience has been that that's about right; my PCs start showing a bit of concern at about the third encounter, and the one time they pushed through to a fifth encounter, they felt very concerned indeed. So call it 4 encounters per game day.
According to the principles of XP, the 4 PCs should level every 13 1/3 encounters. So, that's about every 3 game days.
Now, I have 5 PCs, so it's more like 4 game days--but it balances out.
But what this means to me is this: tonight, I want to add in a little side adventure, because I'm eager to play with my new MasterMaze stuff. But if I add enough action to make that at all challenging, it advances my PCs about a quarter of a level.
This means that just as I figure out what sorts of challenge feel right at one level, they go up a level and give me new challenges.
I should talk about handing out fewer XP. I would rather not get more stingy on my players, but it might be the right thing.