I got pregnant while nursing, but I was 16 months postpartum and still hadn't gotten my period back and was actually trying to conceive (difficult to do if you haven't gotten your period!) So I was actually tracking my cervical mucous (read Taking Charge of Your Fertility, or even simply browse www.fertilityfriend.com for tips on this)and temperature at the time. I pinpointed my first postpartum ovulation with my temp charting and actually conceived our second child on that one, so I never got my period back between children. (and yay for that!!)
The stats on the birth control thing are that if:
1. You are less than 6 months postpartum
2. AND you haven't gotten your period back yet
3. AND you're exclusively breastfeeding (no solids, no formula)
it's about 98% effective. After any of these is no longer true you need to pay closer attention. The cervical mucous is really the best indicator of your fertility and if you're planning on using Natural Family Planning (aka Fertility Awareness Method, FAM) then you should get used to checking regularly. If you really REALLY don't want to get pregnant again, I suggest thinking about a backup. After my second child I had a copper IUD inserted and that's been great so far.