How to get the best from the Canon 100mm 2.8L macro lens?

always use a tripod and manual focus. Live view magnified 10x (if your camera offers it) with MF is the best way to get perfect focus. If I want to get as close as possible, I set the focus as close as possible and move the camera until it is focused.

The teleconverters from Canon (1.4X or 2X) won't work directly mounted between the body and the 100mm lens (not sure about Kenko or other non-Canon teleconverters, they might). You can, however, add an extension tube and then the TC, for example (body -> TC -> extender -> lens).
I don't have an extension tube, but after seeing this, I am planning on getting one since I already have the 1.4X

Edit: Sorry I did not read your post properly, I thought you're talking about the older 100mm F2.8. I don't know anything about the newer "L" version, what I've said in my reply might not be true for this lens.

ok, thanks Diana, Nathan. I have been unsure what to buy, ie, extension tubes or TC, or both.

The Canon teleconverters don't fit the 100L either, but that is not what you want anyway. If you want to get closer, buy extension tubes. I use the Kenko tubes with the same lens you have. They work fine, they are relatively cheap, and they come three to a set, which gives you a lot of possible extension lengths (they are stackable).

My suggestion is that you start with a short extension, 12 or 20mm, and work up to longer extensions as you get more comfortable. The longer the extension, the harder it is to get everything right. DOF gets narrower, light gets dimmer, and focusing gets harder. (You will be using manual focus for all but the shortest extensions).

I agree with both of Nathan's recommendations, if you are doing work where the subject stays put. However, I often find that 5x is plenty (depending on how detailed the surface is), and I often switch back and forth between live view and the regular viewfinder.

Still true, Diana. The 1.4X teleconvertor cannot be used with the 100mm L IS macro by itself.

Martyn, here is the 100 L IS, attached to the 25mm ext. tube, attached to the 1.4X teleconverter, attached to the front of my 7D. With this arrangement, closest focusing distance from front of lens (not hood) to subject is about 4.5 inches. Here's a quick snap of a dime at the closest distance (full image, not a crop).

Wow, I may go for the extension tubes first then and see how I get on with those. If I buy the kenko tubes does AF still work?

I don't want to hijack this post, but Mark, your posting got me wondering. I had never thought to use a TC with extension tubes. If the lens has enough reach, what do you gain by using a TC also?

I don't know if this applies to using a TC as well (I think it should), but without a TC, the magnification provided by a given length of extension is inversely related to the focal length of the lens. So, if this applies with a TC as well as standard lenses, shouldn't the effect of a TC be to bring the object closer but reduce the magnification at the minimum focal distance?

This is what is confusing me also Dan, I think the extension tube allows closer focusing as it moves the rear element further away from the sensor. Then the TC would give you a greater working distance I think.