The most important thing is to make sure your magento version is patched with all security updates.
Assuming that you magento has been installed correctly ( folders and files have the correct premissions etc), then there is other things you can do to be extra safe.
The biggest thread to being hacked (bar having a vulnerability due to out of date version ), is a brute force attack on you admin login. So you can do simple things like change the path of admin login and make sure you don't have a guessable username like "admin" as user name ( so they have to get guess username and password), but even better is block access to you admin area by either having a safe list of ipaddress that can access the admin login area or a have the server ask for a password before it will load the admin login page.
You will find most of the magento vulnerabilities that are found are from having some sort of access to the admin area when you read the security patch release notes, so thats another reason to make sure it locked down well