Preserving biological tissues is difficult. Bacterial decomposition, dessication, and decay begin taking place almost immediately after death. To date, most biological specimens, be they human bodies donated for study or animals collected in the field, have been either preserved in formaldehyde (or similar chemicals), or frozen. Freezing works well from a molecular perspective, but working on the organism is impossible without thawing, and thus decay. Formaldehyde and related compounds prevent tissue decay at room temperature, but they're relatively toxic chemicals, smell bad, and can drastically change the appearance of tissues over time, especially when the tissues are allowed to dry out, as often occurs during teaching dissections.
A relatively new preservation technique is plastination. Plastination involves removing the water and some fats from a specimen's tissues, and then slowly replacing the water and fat with a polymer that hardens at room temperature. The technique was developed by a German doctor, Gunther von Hagens (note: this link contains pictures of dissected human bodies), and is described in more detail here (note: this link contains pictures of dissected human bodies). Plastination is a unique preservation technique: fine details of most tissue types can be preserved, the specimen is dry to the touch, there is no smell, and the preserved specimen can be kept in the air at room temperature without fear of decay or degradation. I have a pre-dissected plastinated frog I show to interested students, and it has stayed in perfect condition sitting on my filing cabinet for the last year and a half.
Probably the neatest aspect of plastination is that organisms can be pre-dissected, plastinated, and then used for educational purposes. Plastinated specimens can be high-quality materials for teaching internal anatomy, potentially removing the requirement for formaldehyde preserved animal dissections in some courses (though clearly some courses' pedagogical goals do require dissections). Since the specimens are durable, a class-set of plastinated specimens could theoretically last years, reducing animal use drastically.
Dr. von Hagens has set up a museum in Germany displaying plastinated humans for public education. While I regret that I've never been to the museum, I've seen pictures from the displays and they're a beautiful combination of art and anatomy. The museum is filled with plastinated human bodies (all donated) that have been dissected and/or positioned in various poses. One of the best dissections I've seen is prominently displayed all over the web site: he's removed the skin from an adult male and has the man holding it in his right hand. On his site he has a few pictures of plastinated humans in the downloads section, as well as in the shop's poster section. I highly recommend looking at the pictures, but be warned that you will be looking at actual human bodies, not plastic models, so if you get queasy at such things I would recommend avoiding Dr. von Hagens site entirely.