Use a good, steady tripod along with a good ball head or a Wimbereley head for long lenses. Place your left hand on top of the lens over the ball head and press down slightly. This helps dampen the vibration cause by the mirror opening and closing. The longer the lens, the more it shows the vibration and magnifies any errors in technique. I used to use a AF300mmf/4 with Kenco 1.4 TC myself, but have since moved to the AFS300mmf/4 and Nikon TC-14E Teleconverter. The difference in focusing speed with my newer setup is amazing.