I usually don't try to sell F# as compared to C# as noted in many other responses here. Since most of my friends work in other languages I usually just use python as my reference.

#ME: "Hey that stuff at pycon was pretty cool, huh?"

#FRIEND: "Yeah it's interesting the directions they're going with the language."

#ME: "Yeah the optional types could help clean up big code bases, and open up the door for quickcheck-like fuzzy testing."

#FRIEND: "Yeah."

#ME: "And some of the functional programming stuff is pretty neat too, I know it's easier for me to test."

#FRIEND: "It's so much easier."

#ME: "And the asyncio stuff is gonna be really nice to work with when it catches on more."

#FRIEND: "Yeah it seems a lot more ergonomic of a solution."

#ME: "And the binary distributable packaging tools look like they might actually work this time."

#FRIEND: "Probably not, but we can hope."

#ME: "I think I should introduce you to my friend F#."

Then just do some stuff in fsx with a type provider or some other neat stuff and at least you've got the message across.

People get excited about solutions to their problems. If you start giving them more problems then you're only adding to the constant subconscious cognitive load that they bear in regards to software development.

If you show it to them in terms of problems that they already have, you're reducing the cognitive load. Everybody loves having less stress.

But this is just how I approach it. F# is like Python4 for me.