You have to commit. Make your eyes wide and when they start to water, squint a bit like you're trying to stop an ugly cry. Then do the chin/bottom lip tremble and take a few shuddering indrawn breaths. When you release the breath, try to discreetly exhale up, into your eyes. They'll water more. Once it starts, it just goes...

But like I said, sometimes it just makes me yawn instead. Like tonight. It's really frustrating because I said I could do it, and now I'm having trouble. :/