Do a Google search for "proper long lens technique." There are some nice articles out there that can explain it better than I can here.

On the old trick of matching the minimum shutter speed to the focal length of the lens (400mm = 1/400, 500mm = 1/500, etc.), keep in mind that if you're using a camera with a crop sensor you should probably consider the "equivalent focal length," (400mm lens on a 1/6 crop body = 640mm = 1/640 shutter speed).

If you're shooting an OEM long lens (Canon, Nikon, etc.) you're probably going to be ok wide open if the conditions call for it. However, if you're shooting something like Tamron, Sigma, etc. you'll usually get sharper images if you stop down one or two stops. Stopping down is also true of using a tele-zoom, OEM or otherwise.

As previously mentioned, use a tripod.

Good luck.