I recently found out about this technique on another forum and thought I'd share it with you guys.
Pretty much this is how it works:
1. Mount a zoom lens onto your camera
2. Take another lens, preferably a compact prime 50mm lens, set the aperture to its widest setting (f/1.8, f/1.4, etc.), set the focus to infinity, and tape the front of the lens onto the other lens on your camera so they are both pointing towards each other, and the 50mm lens mount is on the outside.
3. With your zoom lens, zoom in all the way, set the focus to manual, and get up close to an object, about a few centimeters away.
I find that it helps to use plastic food wrap to cover the glass on the 50mm, just in case you get it into food (like me :P) or you just want to avoid getting dust and scratches on it.
Here's one that I took of my sunglass case with an 18-55mm on the camera and a 50mm f/1.8 attached to it:
Faux Carbon Fiber
More at my Flickr in my signature.