Tubes. I use extension tubes. They do not change your lens optically, but they let you focus much more closely. I use these ones myself:
Vivitar Tubes
Those have contacts for auto focus, etc. I used a set without first, they were $7. The downside is no control over lens settings, but it did not matter much. Canon lenses go wide open when there is no control, I hear Nikon go the other way, so I was able to use them well and focus manually. I focus manually even with the others.