You should be sure that any third-party software packages that you install into your account are kept up-to-date. All plugins and themes you add to such an application also need to be updated regularly. Otherwise, your software may get hacked through a known security vulnerability that has already been fixed in the current version.
The administrative interfaces of your applications must be protected with strong passwords. You can find tips on choosing a strong password here: Password tips. You can also add an additional layer of security by password-protecting the administrative directory of your software (if there is such, and if this will still allow it to function properly for your web visitors) with an extra username/password pair. You can do this via the Protection section of your online Control Panel: Password protection. This method is very effective against brute-force password attacks, and may also prevent certain vulnerabilities in the scripts stored in the administrative directory.
It is a good idea to look for security tips specific to the software that you are using. You should be careful with the information sources though. For example, a badly written security plugin may actually make your application vulnerable.
Be sure that you have no leftover test or obsolete software installations still available over the web. Such an outdated package can easily let hackers into your account. Any software that you do not need should be removed or should have its access limited. You can use the Protection section of your Control Panel to prevent public access to such applications.