If your flash lacks oomph with telephoto lenses, try a flash extender. A flash extender lengthens the reach of your flash by using a fresnel panel to focus the beam to the equivalent of 300mm. When used with telephoto lenses 300mm and up, you get a much more efficient flash that gets substantially more power to the subject than before. On most strobes, the beam of light becomes about 200 percent more efficient.
The only problem with this is that since you are usually further away with a telephoto lens, the light source becomes smaller resulting in harsher light. Since harsh light is a problem, the units are recommended for fill flash only. Try Visual Echoes’ Better Beamer ($39, street).