I'd agree with the comments that battery life for the D200 is not as good as for the D70 and D300 - and other cameras.
Pick up a spare battery - even if it's used. You can often find a deal on used batteries for a fraction of the price of new. Just ask about battery life. Rotate between the primary and the spare.
Third party batteries are not necessarily a good choice - and more of a problem with newer cameras. Nikon branded batteries have circuitry to monitor use and protect the battery from overcharging.
Try to avoid draining the battery completely or to low levels. Top it off after each day of use. Keep it full.
Don't leave a fully charged battery on the charger. Remove it when charging is complete.
Minimize chimping when battery life is at a premium. Using the LCD excessively drains the battery.
Consider turning down the brightness of the LCD by one setting.
Watch lenses that draw excessive power. VR lenses consume power while activated. Hunting for focus draws extra power and can drain a battery quickly.
For me, two batteries has almost always been enough for a full day of heavy use. I have a D200 converted for IR today and rarely run into any battery life issues, but the volume of landscape shots is a lot lower than sports or wildlife. One battery almost always lasts me all day. I just returned from a trip to Death Valley and took 3 batteries but no charger. I used one battery and half another over the week. I did have other cameras with batteries and a charger.