First, don't utter the words "tourist" or "tour." And don't bother hanging around the prison hoping to find a tour guide. That's the easiest way to get scammed. What you'll have to do is call the public phone in the prison (2324033), and tell them you want one of the prisoners to show you around (ask for George Redcloud if you can only speak English). Thursdays and Sundays are the usual visiting days, though bribing the guards (your "tour guide" will handle that) could mean visiting any morning.

On the day of your trip, don't bring any valuables (duh), including a camera, and expect to pay 50-80 bolivars. As if you need to be psyched up even more, here's a cool photo gallery of the place and here's the bestseller, Marching Powder, that was written from the inside. Have fun!