With the exception of the occasional episode of Star Trek TNG, I didn't have much access to much mindfuck sci-fi, so whatever I could get would leave pretty deep impression on me. I didn't immediately read through his stuff, though. I wouldn't really be reminded of it until I got into Sonic Youth in my late teens and found out that the Sister LP was a loose biography. Sonic Youth's guitar rock/pop mixed with discordant feedback was basically the musical equivalent of PKD's paranoia.
After that I spent years going through a big chunk of his catalog. Obviously, a classic like Do Androids Dream of Electric Sheep is worth your time. The Man in the High Castle is alternate history, and features what I still consider to be the best execution of a things-are-not-what-you-think-they-are ever. I've also been thinking of The Game Players of Titan recently, but that's probably because of the talking self driving car. Ubik is also really, really good, but I can't explain it to you at all.
If you want some fiction that will make you question your grip on reality, I highly recommend reading through his stuff. Just don't wait until you're too old - once your ideals are firmly set in place it gets a lot harder to get into it.