By inventing a philosophy, interpreting it the way an intelligent person might, presenting it to the DM, and making sure we were on the same page about what kind of behavior it demands. The DM basically let me write my ethos for myself, and that helps a lot:

My paladin is a librarian. He got into knight-errantry because he needed first-hand experience of the Powers of Darkness in order to write a book. An encyclopedia, really, of methods ordinary people could use to drive back the forces of evil and reclaim their lives. He's going to write it and distribute free copies all around the world in the hopes that one day it will fall into the right hands at the right time. If he does a good job, and has a little luck, one day it might help save the world - or at least some small part of it.

This is important to him because he knows that the universe is and has always been evil.

This is certainly not the fault of humankind, or the fault of his (relatively young) god. It started long before either of them ever existed. Some theologians believe that other divinities may have had a hand in it, but really the blame is beside the point. The crux of the matter is: the universe is here now; we all have to live in it; and evil, pain, suffering, and death were baked right into it from the very moment of its creation. It sucks, but that's life.

But my paladin - and the Order he belongs to - believe that it doesn't have to be that way. The universe can, with unceasing struggle, unyielding vigilance, wise action and constant sacrifice, be made a good place one piece at a time. There are forces for good in the universe. The gods, yes, but more even than them: us. Potentially all of us. Admittedly, some of us don't realize yet that it's possible. Most of the rest are working at cross-purposes. All of us have moments of weakness. Some even deliberately choose to make the world a worse place. A very rare few are so far gone that redemption just isn't possible with the tools we have available. He acknowledges this. But the more of us who can be brought into the fold, the more of the world we can fix.

Thing is, the world is a hostile place. It hates us. It is designed to make people suffer and die, and it is so much bigger and stronger than us it's laughable. So frankly, the light needs all the help it can get. We can't afford to wantonly purge or kill the benighted. They need to be reclaimed if it's even remotely possible. The task of the paladin orders is to go out into the darkness, persuade everything and everyone they can to the cause, and repair an impossibly broken world one tiny problem at a time.

That's how I avoid being Lawful Stupid. By playing a character who is smart, idealistic, allowed by his Oath to use common sense, aware that evil isn't always irredeemable, and wise enough to know that a group of people is stronger together than alone.