The two more effective ways are slotting for perma-dull pain or slotting for positional defense.
Yes Impervium armor has a unique (or is it, Mids is not showing that it is) that grants +3% psi resist and slotting a full set you will get a total of 6.13% resist to psi. Also Aegis set has a dual psi/mez resist that is definitely unique which grants 3% resist ti psi as well. You can also get some psi resist bonuses with 6 of these sets slotted:
Crushing Impact, Doctored Wounds, 3 outta the 4 Fear sets, Decimation, Call of the Sandman, Dark Watcher's Despair, Ghost Widow's Embrace, and so on.
Impervium Psi resist IOs are actually a little over 2% for brutes 
Still, a few of those, plus some other sets will net you a decent amount. Not enough to make you safe, but it'll soften the blow.
My brute is sitting at about 19%, which is good enough with some bonus HP. If I'm fighting a psi-heavy EB/AV, I can pop Elusive Mind for a little more breathing room.
I would like to issue a plea on behalf of Paragon's diminutive protectors, please watch where you step. We're four feet tall in a six foot tall world, we've been cast adrift in a sea of butts. -Pillbug
I love my SS/Inv. Her fury fuelled rampages make even the most mundane missions interesting. KO Blow is great. Foot Stomp is awesome. Unstoppable is... wow that's a killer crash... um, I mean Unstoppable is... um, not quite as godly as Elude is on my /SR. I'll get used to it I suppose.
But... Psi attacks just tear me to bits. I got the Elusive Mind accolade during the last Rikti Invasion weekend, but it only lasts 60 seconds with a 25 minute recharge.
So, are there any IOs that I can slot that will make psi attacks a little less deadly?