I handle commoners a little differently than adventurers. Commoners gain their own kind of XP, "Commoner XP" if you will. Essentially, they gain experience from doing what they do for a living. I don't sweat the details of accumulating this kind of experience (no point really), but those who strive to be the best or are continually challenged in their field will rise above their peers.
Commoners in my campaign gain only skill points, feats and ability score increases as they gain levels, but not increasing combat ability (improving HP, Saves or BAB). Essentially the class is just a vehicle to gain skills, and more closely reflects real life advancement. An "Expert" is simply a high level commoner, so that class is gone. Same with the "Aristocrat", they just have better opportunities to learn a wider range of skills than Commoners. However, most Aristocrates are multiclassed with some kind of adventuring class - reflecting their training with weapons or spells... Most Aristocrates would be Fighter/Commoners, Cleric/Commoners.
Commoners who live in wild borderland areas tend to be multiclassed Warriors/Commoners or Ranger/Commoners. They're farmers, but hunters and militia too... A prosperous farmer who has seen some action over the years might be a War2/Com8. And skills like Profession and Craft are only class skills for the Commoner IMC, all other classes have them as cross-class skills (they can't put in the time and effort into these as the Commoner can). It also makes the Commoner a valued resource in the campaign. The best blacksmith isn't an adventurer, he's a guy who's slaved over the forge mastering his craft all day, every day.
A'koss.
