The most important thing to do is research your subject. Find out when and where they are most likely to be seen. If the subject is not birds, learn tracking methods. Learn how to call game. Most public lands that I know of do not allow game callers, but with practice, you can call some animals in using your own voice (Barred Owls, for instance are pretty easy). If the places you go do allow game callers, use them, but remember your ethics, using game callers too much causes undue stress on some animals. You can learn a lot about getting closer to wildlife from hunting guides. The more you do to improve your chances of getting closer to the wildlife you wish to photograph, the better the chances are youíll get better pictures.
Longer lenses are ideal, but if you canít afford one, a tele-converter can help, but youíll lose a stop or two. Iím not familiar with Nikonís gear, but check to see if they have a converter that allows the older manual focus lenses on the newer bodies. Older lenses are still good and usually a lot cheaper. KEH has some BGN 400 f5.6 lenses for around $400. Use a good, sturdy tripod all the time. Especially with a tele-converter, chances are you wonít be using a shutter speed slow enough to hand-hold and get sharp pictures. Spend lots of time outdoors shooting. Itís difficult to get great shots just by chance.