We waited until DD#1 was 20 months old to try for #2. Like Fletch i needed more time with my current baby......we fell first month (first DTD infact).
Im now pregnant with #3, we fell pregnant when #2 was 10 months. So a 18/19 mth age gap. Would have prefered a longer one (24-30 months i found worked for me), but hey, nature had other ideas.