I like being the one with diagrams and top advice. But Robert over at Zen Films has beaten me to it. He has a fantasticpost on the hard lessons he learnt around promoting his films via social media and using the minimum amount of budget possible. He was promoting two features, but his advice is still true for shorts. How do you get people talking about it. He says what works and what doesn't.
If you are shooting a film now, or thinking about it, or have just finished - read this first - and get your film seen.