Online fraud not only hurts your bottom line, but chargebacks will affect your merchant fees. Since I've been an internet merchant with my site Peggy Li Creations, I've discovered that the best defense is to inform yourself of the warning signs and using the tools your merchant account should provide for fraud prevention. Some warning signs are:
-- Billing address does not match shipping address
-- CVV code does not match
-- Large orders, orders for your most expensive items
-- Orders for items easily resold
-- Requests for items not sold in your store
-- Requests for "rushed" orders
-- US credit card for foreign order(not marked as "International Credit Card" in your merchant system).
-- Customer places a "test" order that processes fine, then follows it up with a much larger order soon after and also orders it expedited.
You should be able to set up the first two checks, or require users to use matching billing address and provide CVV, in your merchant account. It is up to you to decide whether to allow or decline orders that do not meet the Billing Address or CVV code requirements.
Many CC merchant services also provide authorization help -- that is, you can call in for orders over $200 and re-verify the card before charging (and shipping). If I had done this with my recent order, I would have caught that the card had been cancelled before shipping out my goods.
An example of a clever scam -- I had an order from France that seemed legit -- I even got a story about needing items quickly because it was for a wedding (many emails asking for tracking number when I shipped it, when I would ship, etc). I let this first order through, and soon after got a second even larger order from the same customer. However, I got an email with the customer saying she'd like to get additional items not carried in my store, she was going to "ask the bridal party what they wanted..." and hopefully I could "help out." I immediately cancelled the order as fraudulent and told the customer I couldn't help them. I did not hear back at all from this customer since I did so.
Educate yourself on the warning signs! These guys are getting more and more savvy and can cost your business in inventory and, even worse, a bad credit record.
I found a fantastic PDF guide from the folks at PayPal about Fraud Prevention (they are telling you why PayPal is a great solution, but they explain a ton about Internet Safety): https://www.paypal.com/en_US/pdf/PP_FraudPrevention_Guide.pdf