Low Budget Dave wrote:
The general rule I always heard was 1/[focal length]. So at 400mm, 1/400 is a good start. It doesn't mean you have to stay there, it is just a start.
If you have steady hands (I don't), you can rest your hand out near the end of the lens and stop some of the wobble. If you have lowered your shutter speed to keep your ISO down, then consider using the timer. The 2-second timer gives me plenty of time to take my hands completely off the camera.
I am not so sure that rule holds true, with the higher density of today's sensors I find the rule a bit lacking. I do know that for a crop sensor you have to be more careful than with a full sensor with similar Megapixels, I know that shooting with a 50mp full frame is a bit different than shooting with a 20mp full frame.