First, you will need to set up an Amazon S3 account. For the first year, you get up to 5 GB of storage for free, and the pricing afterward is very reasonable, at .14 per GB up to the first 1 TB. Any time you transfer data in, as you will with a backup, it is free, but downloading your data can cost if you transfer more than 1 GB per month.
Once you have your S3 account, you will need to install a plugin on your WP site. Go to your WP admin, click on Plugins, and then Add New. Search for Automatic WordPress Backup and install it. You will need to get the info from your S3 signup for the keys, and then create a bucket, which is essentially a folder for this particular site.
Because you pay for total storage, it might be a good idea to set the option to delete backups older than one month.
Once this is setup on your site, you don’t have to think about it again. Even though you may end up paying a few pennies per month for the storage, it is well worth the peace of mind in knowing your site is safe. You can also use Amazon S3 for personal storage, so even if you don’t have a site to back up, you can still make great use of yet another helpful Amazon offering.