My sims have had four pregnancies which have resulted in two sets of twins. I didn't do anything special as far as I know, nor did I do the same things over the course of the two twin pregnancies (one was a painter with whom I had used money cheats so she spent her whole time painting and the other is a career writer). Incidentally, all pregnancies have also resulted in boys.
The only common thing I can think of is woohooing often while pregnant, but it would be very silly if that was the key.