Compare prices before you buy- There are many comparison sites and tools (such as PriceGong) that compare prices- this can save you a lot of money.
Don't leave the computer unsupervised while shopping online-You don't want someone else will touch anything by mistake and made the purchase.
Log out at the end of the transaction-From the same reason as above- you don't want your information to be public.
Make sure you can contact the store if something goes wrong- You want to validate that you will have someone to talk if something goes wrong..
Always print out your order details and confirmation-For any option that you'll want to contact the site, it'll always better to give them detailed information about your order.
Make sure you buy from a well-known merchant- or check how others rated this merchant, there might be scum sites and you don't want to buy from them.
If you use PriceGong use the merchant ratings, product ratings and product description to know that you are buying the right product at the right store.