I almost always say "My pleasure" whenever anyone thanks me. This is partly because I like saying it (it just sounds better than "You're welcome"), and partly because I said it for 7 years while working at Chick-fil-A (the best Chick-fil-A ever, by the way). It didn't come natural when I first started working there, but now I say it without even thinking.
Well, this morning I had to force myself to say "You're welcome". I held the door open for a lady as I was going into Chick-fil-A for a free breakfast entree. She thanked me for holding the door, and I was about to say "My pleasure", but then I thought it would probably sound weird to her to have a fellow customer say that to her, when the employee at the front counter had just said the same thing. So I managed a quick "You're welcome" before she walked past.
It's funny how much those 2 words are ingrained in me. But I enjoy saying it, so it's cool. :)