Reversing a lens on camera is an inexpensive way to get high quality pictures at higher than 1:1 magnifications.
Most modern macro lenses go down to 1:1 magnification, for example, Nikon Dslr 1.5 crop cameras have a sensor size of 23.7mm x 15.6 so if the lens is set to 1:1 and the object your photographing is 23.7 wide it will fill the whole frame when you take the picture. Now add a 2x teleconverter to the macro lens and set it too 1:1 take a picture of a 12mm wide object and it will fill the frame, your magnification will be 2:1, but since you're using a teleconverter you are losing light and quality, you still will get a good view through the view finder.
Some drawbacks and advantages of different methods of gaining magnification.
Macro lens are great because you focus from 1:1 to infinity, there the easiest way for closeup photography, want some more magnification add a teleconverter, you will keep your working distance and lose a bit of light.
Using tubes, close-up lenses, stacked lenses, reversed lens, all are harder to use than a macro lens but they all produce good results and can be used with a macro lens for getting greater magnification.
Extension tubes, you lose light and the ability to focus at a distance, the magnification effect is more on shorter lenses and less on telephotos, tubes work will with reversed lenses.
Stacked lenses, this is when you use a macro coupler and reverse a lens on the end of another lens, good way to get really high magnifications, working distance is very short and hard to light. I stacked my 28mm on the 200 f4 micro and I was getting 12:1 magnification, it was very hard to work with.
Reversed lens, view finder is dark and short working distance and a single point of focus, very sharp results, compact light setup, especially when used on the lighter cameras like the D70, easy to hold in one hand, on the D200 its harder to hold in one hand because of the extra weight.
Close-up lens, Canon and Nikon make some nice ones, longer the lens greater the effect, working distance is shortened and you lose no light, my Nikon 6T and the 200 f4 micro will get me 2:1 on the Tamron 90 its only 1.4:1, attached to the 70-300 ED it gives almost 1:1.
So why bother with a reversed lens if its such a pain to use, the reasons are magnification and picture quality, and the fact that its the cheapest way to get into macro photography helps also.
Lenses that are suited for reversing.
Prime lenses with an aperture ring like the 50mm, 35mm, 28mm, 24mm, 20mm are ideal, zoom lenses with aperture rings in this range may work also.
approximate magnifications depending of the lens version and make.
50mm lens will give you 1:1 magnification.
20mm will get you 3.4:1
Now one of the great things is it doesn't have to be a lens with the same mount as your camera, I started with a 28mm Yashica lens, it worked great, it was a lens that had been sitting around for twenty years doing nothing, you may have something similar lying around, its a great way to put old lenses to use, ebay is another source and also camera shops, there are lots of old lenses that are good optically and can be had for very little money.
A reversing ring is an adapter with f mount on one side that fits in to the camera and a male 52mm thread on the other side that screws into the front of your lens. Nikon sells a nice one called the BR-2a its a solid metal machined piece that is will worth the money and its even reasonably priced.
You can also make your own from a body cap and a old 52 mm filter, drill a 3/4" hole in the body cap, remove the glass from the filter, glue the filter to the body cap and also fill in the flange with epoxy were the locking pin locks in the body cap, drill a small hole for the pin to go into.
There are also clone reversing rings available on ebay.
Lighting, the working distance is short so lighting can be tricky, I use the built-in flash with a diffuser made from the side of a milk jug, rubbermade cutting board material is even more diffuse and works also.
Your flash will fall off quickly at these distances and the black backgrounds don't look all that great so try to position so you have a background that is close, you can even place something in the background like a leaf, this will give you a lighter background.
Technique, I shoot hand held but for static objects a tripod and focus rail can be used, you will be dealing with depths of field in the less than 1mm range so you will have to practice and come up with ways to stabilize your body when shooting.
Standing up bent over an object generally doesn't work, you body motion will be greater than the depth of field, you will need to lean on something and brace your elbows.
Sitting down is best with your elbows against your knees, start in bright sunshine and set your aperture ring to f8, once you develop your technique and learn some tricks you can use f11 and f16 for more depth of field.
Find a subject, I am mostly interested in insects but flower parts are good way to start since there static and you don't have to worry about stalking, get in position and line up your camera to were you think the subject is, move in slowly, you won't see anything in the viewfinder at first it will all be a blur, keep moving in till the subject appears, look for a fine detail when this detail snaps into focus click the shutter now pull back and do it again, keep at it till the insect is gone or if a static object take lots, depth of field is small so parallel your subject as much as you can.
Depth of field is so small you may want to take some focus layers and combine them later in Photoshop or specialized software like Helicon focus.
Lens hood, you may want to protect the end of your lens, you can take a rear cap for a lens and drill a 3/8" hole, this will also help with flare.
Using the aperture lever, if you want more light for focusing so you can use apertures like f11 and f16, move your hand to the front of the lens and with one finger move the aperture lever, line up your focus then gently release and take the picture, this won't work if you have a lens hood on though.
Stalking insects, to get with in the two inch or less working distance takes some practice, first thing is move slow, no fast hand movements, find a subject that is busy eating, mating, sleeping, sunning, cleaning, if there busy its easier. Find a cooperative subject and don't waste your time chasing one that keeps flying away.
Once you have found one get into your stable position, if the insect is on something I can grab onto I take hold of the object with my left hand, moving slowly, I then bring the object towards the lens, this usually doesn't bother them since there used to being blown around in the wind, your hidden behind your diffuser so you don't look like a predator. I bring me left hand close till it touches the diffuser, I get focus on the subject and with my fingers on my left hand I can rotate and twist the the flower or branch till I get good a position, if the subject is moving around I can move it around to keep up with it.
Here is a depth of field chart I made up, like all the pictures click on it for a bigger version.
Reversed lens work is challenging, requires practice and technique, but its very rewarding you will get sharp high mag pictures that are second to none.