Nikkor 18-200mm VR ED Lens Zoom?

I'm a new DSLR camera user...learned many things by past few months..one thing I'd really like to know that are there any particular good settings to take snaps in more than 100mm...say like 150mm or even at full 200mm(through a Nikkor 18-200 VR ED Lens),Camera I'm using is Nikon D200.Whenever I go for longer zoom say above 100...150..or 200..the sharpness of images reduces a lot.I tried with all focus points as well as tried different aperture.Are there any settings or combination of settings like Shutter Speed+Aperture+ISO etc through which I can take sharp images of far objects using maximum zoom(200mm) or it is by default that ,as I zoom higher...the sharpness also starts decreasing.