Yep, extension tubes are the way to go if you want that real close up magnification kind of shot. I hear that kenko make some good tubes for Canon/Nikon.

For best results you want a 85mm or longer prime with a nice and large aperture (f/1.8 or less) and an extension tube for that close-up, shallow DOF effect that most macro photographers desire.

With what you have the telephoto zoom is about the best you will be able to do, as the others said.