Mumtoa, I would just sit down and wait. I've done it in the middle of a shopping centre, just found a seat where ds could see me, and watched and waited. Eventually he calmed down and came to me, I blew his nose (signifies the end) crisis over.
My poor kids, though, I find it really hard to take them seriously when they're cracking up. Luckily it doesn't happen much.
GL