Some exceptions are while driving (but who doesn't have a little road rage?), or if I'm extremely frustrated, or if I hurt myself. Although even when I injury myself, I tend to use PG-13 rated curse words instead of R.
In French, however, I'm a regular ol' sailor.
I think this is is because of the following reasons:
1)The P word and the M word are foreign, and
2)French people don't take their gros mots any where near as seriously as we do.
They can say merde on TV for crying out loud.
If you want to speak French like a French person, I suggest you become a bit more comfortable with sparkling your every day language with their two most popular curse words.
And, if you are worried about overcoming the giant obstacle that is speaking French, worry no longer. Brooke at skymachines has discovered a French language video I find to be extremely helpful for dealing with both problems at once.