I like this from The Penguin's blog:
Sometimes, though, if I go to a race I’ve never done before, I don’t see any of my old friends, so I’ve developed a system to help me make new friends. This system has been thoroughly tested at running events around the country. I pass it on to you for your use.
Never try to talk to someone who isn’t wearing socks. I don’t know why, for sure, but people who don’t wear socks also don’t talk.
Don’t try to talk to anyone with a tattoo on his or her ankle.
If the temperature is below freezing, don’t talk to anyone who is wearing only running shorts and cotton gloves.
Don’t talk to anyone who is wearing a shirt from an impressive event. They want to tell you about it.
Find someone who is standing alone near the back. They haven’t done many races and will welcome the company.
Look for people near the back wearing new shoes.
Look for someone wearing a shirt from some other sport, like a professional bass fishing tournament. They’ve got stories to tell.
For races over 5K, get into a sprinters crouch. If the person next to you looks over and does the same thing, they know less about running than you do – and you’ve found a partner for that race.