Originally posted by RioRico 5: The simplest way for ultra-macro is with reverse lens-stacking. Stack a 30mm prime on a 300mm prime for 10x magnification. Working distance is ~45.5mm.
Just to add to that, don't expect to get much in the way of usable images, unless your subject is extremely flat (like a postage stamp). I got to about 8:1, and it's very difficult to handle, and even at 5:1 the DoF is about as thick as a sheet of paper with the aperture open to focus, and not a whole lot better stopped down.
One thing my experimenting has taught me is that I need much less magnification than I thought I would, to shoot most of the subjects I like (e.g. bugs, flower parts, lichens, etc). 1.5:1 is plenty for most things, and I wouldn't want to go much beyond 2:1 for those.
Now, if you get into focus stacking, the game changes. You can use much higher magnification and shoot many "slices" of the subject, merging them in software to keep the focused parts.