This little article is based on a house rule from my current Pathfinder campaign. I originally found it in a Greyhawk brazilian site some years ago and used it in many D&D games. That was before prestige classes like the mystic theurge. Unfortunately I never found the site (or its creators) again to give them proper credit. Please, bear in mind that this house rule greatly improves the power of spellcasting classes (which is fine by me, since I see that rather as a feature than a “bug” - in other words: I like powerful spellcasters in my fantasy worlds).
Caster Level Progression and Virtual Caster Levels
Like the Base Attack Bonus progression, all spellcasting class keep improving their caster level while multiclassing in non-spellcasting classes, but they are susceptible to a special limitation.
For every two levels in another class, a spellcaster increases his caster level, spells per day and spells known (if applicable) by +1. This works exactly as the “+1 level of existing spellcasting class” feature of many prestige classes. These “ghost levels” of spellcasting are called Virtual Caster Levels (VCLs).
The maximum number of VCLs any character may gain is equal to half their real caster level (round down). For example: a 6th level sorcerer could gain a maximum of +3 VCLs by multiclassing in other classes. Some levels later, the now 6th sorcerer/4th rogue (a 10th level character) would cast spells as a 8th level spellcaster. In the future, as 6th sorcerer/8th rogue (a 14th level character) his caster level would be 9 (6 from his sorcerer class, plus 3 from VCLs).