I had twins and triplets yesterday (both first generations, two different families). I think the key is the younger the couple the better the chance, as that is when it seems they are more fertile. Also, they were full of energy, hunger, and were in happy mood. I gave them both showers before and everything...it worked.

(Both I had JUST adopted, given the adoptee a shower, accepted the first marraige proposal, and they went straight to the bedroom...no celebration. Came out with multiples.)