I like to use my long telephoto lenses as much as possible, especially for landscape photography. However, it can be a real challenge to get sharp images in certain conditions. The problem is wind-induced vibrations, which becomes all too apparent when using lenses with a focal length in excess of 300mm (especially large fast aperture lenses).
The main cause of this problem is normally an inadequate tripod collar. These lenses haven't really been designed to take landscape images using relatively long exposure times from a tripod, and consequently their tripod collars just aren't up to the job. Once attached to a tripod you can normally feel and see the amount of play in the tripod collar. As the whole weight of the camera and lens are supported from this single point, any movement is bound to cause and amplify vibrations as soon as the wind blows. Using the image stabiliser can help to reduce this problem, but only up to a point. The stabiliser can't be used in combination with long exposure times, as the image will drift during the exposure, causing even more image softness. Using mirror lock-up and a remote release isn't the way to go either - they might help to eliminate mirror slap but they do nothing to prevent wind-induced vibrations. In fact the effects of vibration can often be reduced by dampening, and for this reason I sometimes brace the camera against my face and drape my arm over the lens, which can reduce the vibrations considerably. Obviously this is still not a solution where very long exposure times are concerned. The best solution in my book (where possible) is to support the lens on two heavy beanbags.
I place one under the camera body and another under the lens hood, creating a support at either end of the camera/lens ensemble whilst not fouling the focussing ring. I fill my beanbags with birdseed, which provides a really stable support. An alternative solution that, whilst not as effective as beanbags, can still eliminate some vibration, is the Long Lens Support Package available from Really Right Stuff.
By supporting the lens in two positions this handy accessory can really help you to achieve sharp images when using longer focal length lenses in windy conditions. It is not cheap but it is at least adjustable so that it can be used on a variety of longer lenses, including zooms.