The only way to repetitively increase potential is with the Ultimate Spell and the Mana Siphon spell.
In general, potential is an innate trait, and increasing it over a lifetime should be difficult or impossible ;p
I also recommend taking a look at the FAQ.

if u have a lvl2 source and marry another lvl 2 or even lvl 3 source good chance to get high potential hopefully i love it bcz when i get a powerful source character i make sure he or she reaches there full potential id even go as far to duel with them over and over to increase there combat magic lvl so they are less likely to die in a magic duel against a random scrub who doesnt even come from a arcane family ;)

If you're looking to increase your source potential and you got the basic source trait from Mephistopheles or Hecate, the best way to level it up is to recruit minor demons using hell's scepter. They all have the source trait, and if you dominate them you can imprison them pretty easily (imprisoned characters can't refuse mana siphon). From there you can just mana siphon demons every year to get your source trait to level 4, at which point the only way to level it up to Chosen One is through the Mage Summit.

Wonderful mod! How do I get the Mana Siphon spell? Is that new since the update?

@cirjuly85
This is nothing new. It's an advanced spell from magical research (the option to develop powers and then restore powers, if I remember correctly).