Those expressions are tough. Hmmm. Where I live (southern US), we wouldn't put the "had" in example one--I'd say "I used to think that...." But, to be on the safe side, you might want to avoid example one in writing. "I once thought that..." is direct and concise. Example two is fine, and I can't think of another way to write it (well, maybe "I had been accustomed to thinking..."). Anyw