In my campaign, which kicks off this Sunday, I will require characters to train before leveling. I'll inform players when their characters are qualified to gain a level, and at that point they'll need to take some time to either work on their new techniques or find a mentor to help them. If they decide to train alone it will take a number of uninterrupted weeks equal to the level they are advancing to before they can spend skill points, choose feats, etc. They can halve that training time by learning under a mentor. I chose this method of advancement for three reasons:
1. To spread out the pace of the campaign and allow seasons, years and events to pass as the characters grow.
2. To make at least a passing nod at realism. You just don't go grasshoppah to grandmastah in six months.
3. To create more opportunities for neat roleplaying encounters. When the PCs choose to seek a mentor it will require roleplaying and perhaps even unplanned adventures. As the PCs level up, finding new mentors will become increasingly challenging, prompting unplanned trips to remote, exotic, or even dangerous locales.