Hiya!
[MENTION=29398]Lanefan[/MENTION] ahhh...got it. I didn't read the whole thread, obviously.
(dusts off "Dunce" hat and places it on head for the rest of the day)
In game-time, if I was so inclined, perhaps bringing in the 'training rules' for when a PC has enough XP to gain a level could be used? I'm thinking of the 1e/Hackmaster4 method wherein once a PC has enough XP to gain a level they stop gaining XP until they "go train and contemplate" what they have learned...by spending GP's and time. I never really used them because, well, my Players PC's don't gain levels very quickly and it's never bothered us. But in the 1e/HM4 rules, the DM assigns an overall rating (1 to 4; 1 as Excellent, 6 as Poor), taking into account pretty much everything the
Player has done at the table. Roleplaying, problem solving, tactics, pretty much everything. That number, times the characters new level, is the number of weeks it takes to 'level up'. It also costs, iirc, some ridiculous amount of GP's to pay for everything during that time; like, 1k gp x Rating x Level or something. I never used it...too much of a "downer" for the excitement a player gets for leveling up his PC. Kinda like graduating high school.

(
"Yay! No more school! Finally!...crap... now I gotta get a job and pay my own bills..." 
).
Here's an idea that just popped into my head...
"Delayed XP Gaining"
The idea is that a PC is constantly doing stuff (gaining XP) but needs to rest, relax and reflect (call it the Three R's of Adventuring). Pick some number that suits your 'advancement rate'; I'm starting with 100xp/day.
Now that we have the Rate of XP gaining for the Campaign Setting (100xp/day), that's how many days of "non-otherstuff downtime" is needed to
actually get the XP's earned while adventuring. How it would work is that DM calculates XP as is normal for his game. He then divides that by 100 to get the number of 3R's worth of days the character needs to spend, well, doing the 3R's. He can't spend this time doing other "downtime" activities (as per DMG).
EXAMPLE: Bill the Barbarian is 3rd level. He has been adventuring for a couple days, clearing out an infestation of nasty goblins that have been doing far to much goblinry far to close to town for far too long. The last big push by his group and him netted them each 800 XP. This puts Bill up to level 4 now. Taking 800, dividing by 100, gives Bill 8 full days of 3R activity to perform before he actually gains his new 4th level perks.
As the character gets higher and higher levels, it takes more and more time. So when a PC gains 34,200 XP from a massive adventure out in the Ethereal Plane, it's going to take him 342 days (!) to actually gain that new level. If that's too long, put in some other number for the 3R divider (maybe 1000/day is better; that would take 34,200xp down to 34 days). I'd just make sure to keep it consistent once a number is chosen. Consistency is key as a DM.
^_^
Paul L. Ming