Like for the bear hug picture, which also went viral, I just happened to be in exactly the right place with exactly the right lens, which doesn’t always happen. And sometimes, embarrassingly, I don’t even notice that something is happening until afterward, like with the kissing kids picture that Gawker covered. I would love to say that I saw that happening, but I totally didn’t.
Finally, cast a wide net. I’m not saying to lean on the motordrive, which I think is kind of weird and alienating (you don’t want your subject to feel like they are being machine gunned), but the more pictures you take, the more often you are going to get something great.