I feel this has been an often brought up topic but my search foo failed.
As a caster, what's the best ways to gain constant DR? I'm looking specifically to avoid stoneskin for longevity and reliability reasons. Is there a best mix of Paizo races, feats, non-armor gear, etc. that accomplishes this?
Aasimar can take 2/magic or 2/evil instead of their SLA.
Defending Bone gives 5/blunt.
You could play a werewolf, but keep in mind that Lycanthropes include the CR of the base animal in the template adjustment. So werewolves should be treated as a +2 template.
There are probably more, but I don't remember them.
Most DR-granting spells are either 1 minute/level polymorph spells or 1 round/level buffs. Ablative Barrier is hours, low level, and kinda-sorta DR.
I was going to recommend the Ring of Continuation, then I saw that it has been nerfhammered in the FAQ (and good riddance, what the F... were they thinking?).
The answer seems to be "play a druid".
Teifling you can take 2/ silver, 2/ cold iron, and a few others like the Aasimar it replaces the spell-like ability. There are also the alignment templates which are an ECL of +1, also the Fey Creature (+1), Half-celestial, and Half-fiend (both +1-3 depending on level, PCs we usually use +2). I play alot of monsters, mostly fliers.
I knew about the Aasimar and Tiefling options but found them generally lacking. Thanks though! DR 2 is just meh over the long haul imo since DR doesn't stack. Thanks for the Invulnerability option. I didn't know about that. It gets the kind of DR 5/10/+ I was going for. Thanks Magda.
Make it a Haramaki or a Silken ceremonial. No spell failure.
Also Chillblight Emissary trait gives DR 1/cold iron.