Do your homework
If you are buying something over the telephone or internet and want to use your credit card, make sure you know and trust the other party. If you want to provide your credit card details to a telemarketer, take their name and call them back on a phone number you find independently (i.e., not a number they give to you).
Check over your credit card and bank account statements as soon as you get them so that if anybody is using your account without your permission you can tell your bank.