Make sure things are well lubricated, as this prevents the condom from ripping.

If you use a lubricant, be sure it's water based; silicone based lubes degrade the latex in condoms.

Check the expiry date frequently, and before you buy them.

Don't skimp on brand. Buy Trojan or similar brand.

Buy the correct size. Condoms that are too loose pose the risk of falling off, or fluid exchange. Durex is known to be good for smaller dudes, and magnum for vice versa.

Don't keep them in your wallet or car! Risk of perforation, etc etc. Condoms should be stored in a cool, dry place; in a nightstand is perfect.

Hope this helps! as long as you use and store them properly, most are electronically tested for effectiveness. :)