I have two close friends and two family members, all Americans, who have spent time in Iran in the last ten years and all of them describe it as a warm and friendly place for an American to travel. If you can manufacture any kind of "official visit" rationale, that'll help with the visa. I mean, if you can make contact with the Tehran Cycle Club and get them to invite you for an evening's bike-cultural exchange, that'll help -- anything more formal will help even more (best of all are scholarly invitations, so long as the context/theme is apolitical).
If my friends' experience is any indication, you're more likely to get harassed by the US government when you return than messed with by anyone in Iran (one of my friends had to politely decline an invitation to explain her trip to unspecified officials when she returned; they didn't persist, but somewhere in the bowels of the American security apparatus is a file folder with her name on it . . .).