Turning someone down

How do -you- turn someone down?

I'm not sure how many people here have had experience with such a situation, but even if you haven't, what do you think you would do? Share your experience, thoughts and stories, from either yourself or someone else. I'd love to hear them.

Declining an invitation to go somewhere? Rejecting someone who just asked you out? Declining an offering of a cigarette when you're trying to quit? You can turn someone down in a number of different scenarios.

A simple "no thanks", "I don't think I can do that" or other similar phrase possibly catered to the scenario would be sufficient in all three I would have thought, just might require guts though. Not speaking from experience.