Go to the Encampment, and standing next to the Riftstone in the back of the Innkeeper's tent is this elderly pawn by the name of Johnathan. For a fee of RC, he will sell you Inclination potions.
If you're short of RC, buy 3 Challenger and 5 Utilitarian and drink all of each type in the reverse order you want them expressed in your pawn.
For a Sorcerer, I like running Utilitarian/Challenger/Mitigator. For a Ranger, I like running Challenger/Mitigator/Utilitarian.
The inclination forcing trick I found while playing DD doesn't seem to stick as well in DA due to several things, primarily because of all the vocation switching during purifications. I reestablish my Ranger main pawn's existing inclinations after purification vocation switching with one Challenger potion and that seems to keep him stable. The forcing trick I initially found was drinking fifteen each of the three inclinations you want expressed (yes, the tertiary has an affect on your pawn's behavior) in a group, in the reverse order.
So, if I want Challenger/Mitigator/Utilitarian, I now drink 11 Utilitarian, 13 Mitigator and 15 Challenger. That reinforces the behaviors I want from my Ranger, which is shooting that bow more than dagger use-- just like I trained him to do. It's also why I don't equip a dagger skill ring on him, because it overrides all his training and those inclinations.