You can always use Harry Potter style swearing. There's a lot of, "Harry swore under his breath," in those books, but that only really works for interjections.
I've heard it argued that, for example, a prison scene wouldn't feel realistic if the prisoners weren't swearing, and their dialog should be written accordingly. I'm not looking for realism in all aspects of a story, though; I'm reading to be entertained. I don't ever like to read profanity, but I tolerate a certain amount of it. Regarding Superman in particular, I agree with others' sentiments that I would find it out of character, and it would detract from the story for me.
Personally, I wouldn't feel comfortable writing characters using language in a story that I wouldn't feel comfortable using in conversation myself.
