In order for a condom to be effective, it must be used before any type of sexual encounter/act, from beginning to end.
Do not "double-bag" (using more than one condom and one time) they could both tear due to friction between them.
In order to prevent the transmission of HIV (the virus that causes AIDS), you must use a latex condom, and not a "natural" or animal skin condom. Animal skin condoms have tiny holes that HIV can get through, whereas latex condoms do not.
Store condom in a cool, dry place. Stay away from storing it near sharp objects or in the heat. That includes wallets.
Condoms & STDs: CDC Prevention Messages
The following are prevention messages recently developed by the CDC:
Latex condoms, when used consistently and correctly, are highly effective in preventing the transmission of HIV, the virus that causes AIDS. In addition, correct and consistent use of latex condoms can reduce the risk of other STDs.
Latex condoms, when used consistently and correctly, can reduce the risk of transmission of gonorrhea, chlamydia, and trichomoniasis.
Latex condoms, when used consistently and correctly, can reduce the risk of genital herpes, syphilis, chancroid, and HPV only when the infected areas are covered or protected by the condom.
The use of latex condoms has been associated with a reduction in risk of HPV-associated diseases, such as cervical cancer.
Latex Condoms and Sexually Transmitted Diseases - Prevention Messages, National Center for HIV, STD and TB Prevention, Centers for Disease Control and Prevention, Atlanta, GA (undated document).