This isn't about how to get a lot of shots per charge. That's easy: use your LCD as little as possible and spend as little time as possible in the menus or with the meter on. Turn the LCD review OFF while shooting. Easy.
This is how to get years of great service from expensive batteries. Some people always seem to have problems while others have them last for years.
I've made 24,000 shots on my D200 in the past 8 months. I use two batteries in rotation, and each still reads "new."
I always have good luck with batteries. My Nokia cell phone is from the 1990s and its lithium-ion battery is still working as well as it did new. I've charged it well over 1,000 times. I used my cell phone all day (1,500 minutes a month) for most of those years.
Ditto for my 3-year-old Apple iBook laptop. It still runs like new. Each charge runs for hours and hours.
HOW DO I DO IT?
As you can read on my battery page, Li-ion batteries need a very different kind of care than the old, heavy Ni-Cds and Ni-MH batteries did.
These suggestions only apply with Li-ion batteries. These are the lightweight ones that are custom-fitted to your phone, laptop or camera. They have weird voltages like 3.7V or 7.4V. These are the dedicated batteries used in all modern digital SLRs like the D80, D200, D2X, D100, D50, D70s, Canon 20D and any point-and-shoot that takes a dedicated battery. Hint: these batteries will be marked "Li-ion."
This does not apply to ordinary AA or other standard size rechargeable cells. Those are Ni-Cd or Ni-MH and want to be 100% discharged before they are charged.
I've never bothered with off-brand batteries. I always use batteries from the original maker.
Modern Li-ion batteries love to be charged often. They don't like being fully discharged. This is completely opposite from older style batteries.
I charge my batteries every night. My cell phone goes on its charger at night and comes off in the morning.
Alternate Between Two Batteries
I have two batteries for my D200.
After every use I put the battery on the charger. I take the second battery from my case and put it in my D200.
When the first battery finishes charging I put it in my bag as a fully charged spare.
I repeat this continuously. This way each battery is used only half of the time and I always have a full battery in my camera and a fully charged spare in my case.
I don't bother if I've used it for only a few shots or used less than 10% of the charge.
It also means that I never try to suck either one dry. Li-ion batteries prefer frequent and shallow discharges. If you do that you'll get far more total energy out of them.
I don't know the exact numbers, but you'll get about four times as many charge cycles out of them if you only use 50% of the charge each time, and twenty times as many cycles if you only use 25% of the charge each time.
Even without a second battery I've gotten great life with frequent charging. My D70 only has one battery, and after over two years and tens of thousands of shots it also runs like new.
Camera batteries have come a long way in the past few years. I've not had any problems, just loads of power. Li-ion are the best: they weigh very little, they pack loads of power and they don't need any babying.
Charging Li-ion batteries is like lunch: do it early and often.
The old heavy Ni-MH batteries from the D1X were awful! They were an entirely different technology. Good riddance! I still use Ni-MH as AAs in my flash.
If you find this as helpful as a book you might have had to buy or a workshop you may have had to take, feel free to help me write more with a donation.
Thanks for reading!