Well actually, to add to StrawberryFrog's suggestion about the broomstick, there is archaelological evidence of certain cultures — among them, the Mayans — using ritual enemas to inject a psychedelic tea in through the out door, if you follow me. The brew was made out of psilocybin or peyote or whatever was the drug of choice, placed in some sort of bladder with a nozzle, and administered. Whereas the broomstick story sounds like an urban legend, they have actually recovered these enema devices. They look like turkey basters.

Given that one of the large intestine's main roles is to remove water from whatever is passing by, these psychedelics easily pass through the intestinal walls and into the blood stream. The best part of this method of ingestion is that the user/shaman doesn't get sick. If you've ever done mushrooms, you know what a challenge it is to keep the dang things down. Of course, the down side is that this technique probably won't make you too popular at parties.

See also: peyote enema.

I would node some specifics, but even though Google turns up a host of sites when you search for "psilocybin enema drug anal," the damn firewall here at work won't let me open them. :)