I first got these latex dental dams from the "Sexperts" group at my university. They gave these away as well as condoms and lubricants as freebies at a Sexperts event. I have to say, if you are going to buy dental dams, these are the ones to get.

Dental dams protect against STIs while performing oral sex. The could also be used to protect yourself while giving a "rim job" They are nice and stretchy, so all you do is stretch the dental dam over the area you are going to be coming in contact with. These ones are flavored, so you do not have to put up with a gross latex taste in your mouth and inhaling the scent of latex.

These dental dams are rather cheap, but if you cannot afford your own, you can make them. Cut down the side of a condom so that you have a rectangle, and voila, you have created your own dental dam. Also, non-microwaveable plastic wrap (but make sure that the plastic wrap is in fact non-microwaveable, or it will allow fluids to still be exchanged).

Many people do not use dental dams during oral sex since they believe that they will not exchange STIs through oral-to-genital or oral-to-anal contact. It is still a possibility, so protecting yourself with a product such as this is a good idea.

Also, oral-to-anal contact can turn some off, while being an exciting thing for others. If the thought of sticking your tongue in and around your partner's anus makes you uneasy, this can allow you to do that while 1) keeping your mouth clean, 2) protecting yourself from STIs, and 3) pleasing your partner.

These dental dams should be carried around as often as you carry condoms around. You never know when you could be having oral sex, and you want to protect yourself when that time comes!