if you havn't got a cable release get one especially for tripod use or just resting the camera on a bean bag there's no need to touch the camera avoiding even the lightest camera shake
i use a bean bag all the time and a wired remote shutter release and have good steady shots at less than 100th of a second at 400mm f5.6
example taken at 398mm f5.6 1/50th sec iso 200
i can't remember if i used a tripod or a bean bag for this shot but the remote shutter release allows me to get quite steady shots at low shutter speeds
i try to keep the shutter speed above 200th second and the iso as low as i can get away with but unless you have fast glass ( i havn't ) it's not allways possible anything that can aid your photography has got to be a bonus and a cable release has got to be one of the most invaluable things you will buy
another example taken at 453mm f6.3 1/60th sec iso 320
this was taken at 328mm 1/13th sec f6.3 but i think thats pushing it a bit
bit of a long winded post but i hope some of this helps in some way