#1 - Go over ALL charges on your credit card bill.
#2 - Shop at secure AND trusted sites only. This means they are using https: with a valid SSL certificate AND that they are trusted. Trust is relative so lets say you find an unbelievable price on some website you don't recognize but Amazon.com is still about $15 more. I'd recommend using Amazon because they are a known brand and that $15 you may have saved on the one transaction could turn into a loss of hundreds or thousands!
#3 - Use one time credit card numbers. More details on these are at the link below but the companies that provide these still provide you the protection of the credit card itself unlike pre-paid debit cards.
Details on each of these items can be found at the F-Secure Safe & Savvy Blog