Some people have a pretty good idea of how to lead by example...
I have to admit I tend to swear more often than not. Not all the time mind you, I know my place. But around friends and male coworkers, I've got a pretty nasty mouth.
The problem with that is that you expose yourself to slipping up in the wrong surroundings. I do it with my daughter all the time (4 years old). Once at the beach with my wife and her, we were merrily sunbathing, eating our homemade sandwiches, and playing around in the sand. There were many seagulls around, waiting around for any discarded bites of food. But they were keeping their distance. Not a bother at all, really. Until my wife decided to throw them half her sandwich. Then they just crowded in around us. I couldn't believe it! In a flash of temper I yelled out to her: "Don't feed the fucking birds!" The instant I let it out I knew it was bad news. And sure enough, not ten seconds later, my daughter points at another nearby flock of seagulls and says: "Look Daddy! Look at all those fucking birds!" Naturally, both my wife and I nearly died laughing. But it showed me how easily and quickly they pick up on our bad language.
She hasn't said it again since (partly because we made it clear it wasn't a good word!) but I'm sure if I continue to use it in front of her she'll begin to think it's okay. The best policy is to try and stop using foul language altogether, so I'm going to work on that. Wish me luck!