Another sneaky tip and a last resort after applying all the above good advice is to put the camera into burst mode and fire off a few frames (usually 2 or 3) this can sometimes help a little as when you are first making the motion to press the shutter release your are introducing movement and vibration. You may find that the second or third frame is sharper due to there being a touch less vibration.
There is no substitute for good technique and knowing your own and camera limits though.
