Never reuse a condom. Don't use old condoms or ones that have been exposed to heat, such as stored in a wallet for a year. Don't lubricate the penis, other than perhaps a little on the tip, before putting the condom on otherwise it is too easy for it to slip off. Large size condoms are a lot easier to roll on if you have an unusually thick penis. The rolled condom is not very elastic, though the unrolled part is very stretchy.
Experienced users have far fewer condom failures, both breaking and slipping off, than novices.
Failure rates for individuals vary by factor of 10 depending on their sex practices. In the literature there are a few fluke cases of transmission by kissing where both parties had bleeding gums, but none for water sports, mutual masturbation, massage, or sharing cutlery. Diaphragms and female condoms are not as effective as traditional male condoms. It is too easy to spill the semen. Even though modern retroviral therapies can reduce the levels of HIV in the blood to undectable levels, it is still present in semen in about one third of men, even after six months of therapy.