We didn't plan to have our #2 daughter, but we found out we were pregnant when our #1 daughter was 9 mos old. SURPRISE! I was worried too, but, to use a very tired analogy, it was just like riding a bike: I didn't forget what to do. Also everything has been easier the 2nd time around: delivery & recovery was easier, BFing has been easier, knowing how to care for the baby has been easier--its easier because you have a clue! :-)
By the time #2 arrived, #1 at 19 mos was walking and was pretty independent (she could play by herself for a little bit, feed herself, etc), which made it easier when I had to tend to the newborn. The first month with both girls was the hardest and the hardest thing to adjust to was not sleeping. But once #2 started sleeping through the night, things have been much much easier.
I am very happy to have both girls (despite the fact that I'm referring to them as numbers right now) and I'm glad we didn't plan it. The girls get along very well (now #1 is 24 mos and #2 is 5 mos) and #1 likes to entertain #2. And #2 loves to laugh at #1. I see them becoming fast friends as they get older.
There really is no "right time"; you can help things along (more mommy/daddy time, no birth control) and just let mother nature decide for you. But once the decision has been made, you'll be ok and you won't forget.