In a casual environment, you could wear a shirt that tries to convey the message. Note: the link is just one example of a shirt that should work. There are others out there, I just don't remember them off-hand.
There are also buttons with similar messages that you could get and put on your backpack, suitcase, or whatever. I apologize, I don't remember any of these off-hand.
I've not actually used either of these techniques, but I know people who have. The shirt in question is one a friend of a friend wears on days when she is feeling more social. When she wears that shirt on public transportation, random strangers will talk to her, regardless of whether she's wearing headphones or not.
The button idea is from another friend of a friend. She keeps six buttons on her backpack. Three of those remain there at all times, the other three get changed out based on her mood, and a few of those are various degrees of inviting people to talk to her.
Neither of these friends of a friend will have conversations with random strangers on every outing they make while decorated for socializing. But they both said it happens a lot more with their social dress on than without it. It's also more effective than anti-social dress, which they've found can also cause people to try striking up conversations with them.
In addition, I've seen other people around using these techniques on public transportation. It's not common, but it happens, and I'm not the only person who talks with them.