I'm a mom of 4, and still nursing, so I hope I can offer some help. I nursed my twin girls for 3 years, and I'm currently tandem nursing my 21m son and my 6m son. I've had lots of different nursing experiences along the way, and there are so many resources out there to help us, so hang in there! You're doing a fantastic job!
The number one thing is (obviously) that nursing is a demand and supply. The more that's removed, the more you make. That can be tricky for so many reasons of course. (baby distractions... growth spurts, introduction of solids, mom's hormones, mom's diet and energy level, other meds, etc...) so every situation might have its own remedy.
kellymom has a tremendous amount of advice for nursing issues and seem to be the most reliable in terms of research. Moms at local la leche meetings (http://lllusa.org/groups.php
) also offer a huge support network, as there's always someone who has been there and can share first-hand experiences.
And I totally agree - pediatricians will be the first to say 'introduce solids (or formula) simply for liability reasons, or to say to the mom 'you've done enough...' but that doesn't really help the problem, only eliminate or go around it.
My twins were (and still are) total lightweights - they were always at the bottom of the charts. They nursed so often that I was sure my supply was the problem so I took fenugreek and pumped like crazy. Ended up with an overabundance, lots of plugs and mastitis. I think I went a bit overboard.
As for nursing a budding toddler and keeping your supply up, always nurse first - first thing in the morning, before any snacks or meals, before naps, after naps, before bed, (during the night if baby wants it). And offer. Don't wait until the tell-tale signs. After all, nursing is so much more than food.
As for pumping to increase supply, that does help but don't pay too much attention to how much is coming out. Pumping is not nearly as effective as a baby suckling to increase supply. And the frustrating things about a weight check before and after nursing is this: what if your baby was too distracted to nurse a big meal? what if your baby wasn't really that hungry, but just a little bit thursty? what if your baby was sooo hungry, that she got frustrated and didn't nurse well? what if (fill in the blank) - there are so many reasons...