The posts describing the reversed lens technique are located here and here. The post describing the extension tube alternative is located here.
While my 300mm f/4 IS lens is good for shooting insects and flowers as it focuses to 4.9 feet, there are ways to get greater magnification out of the lens. Last summer I played with stacking two 1.4X Canon Extenders with a short extension tube between them. You can read about that experiment here.
I picked up a Canon 500D closeup lens for the 300mm f/4 over the Winter. This changes the focusing range of the lens, which is from 4.9 feet to infinity without the closeup lens, to from 17 inches to 25 1/2 inches with the closeup lens.
Below are four shots, the closer two having been made with the closeup lens attached and the other two being made from about five feet from the subject without the closeup lens. All shots were made with the Canon EOS 5D Mark II and EF 300mm f/4L IS USM lens at f/8 and ISO 800.