I managed to contract HIV myself in the summer of 1985 with one delightful night of unprotected anal sex with guy who looked healthy and well-muscled as the proverbial horse. A condom exposed to heat or more than a year old will easily break. Don't carry a condom in your wallet. See the gay glossary under safe sex for more information. Use lots of lubricant, water based , such as KY, Mukko jelly, Probe, Elbow Grease etc.
Some sex lubricants come in both water and oil based versions.
Make sure you get the condom-friendly water based one. Oils and Vaseline rapidly weaken a condom. You can put lubricant both inside and outside the condom, but don't put it inside other than on the tip, or the whole thing can get so slippery it will pull off. The screwee can't tell if the condom is in place on not. If the screwer notices a sudden increase in pleasure, chances are the condom has come off or broken.