Physical Security :

The system console should be located in a secure environment (locked room, cabinet, etc.) where only authorized personnel are allowed.  If there is physical access to the computer, it is easy for a customer to break-in or misuse the system. 

Do not leave the console logged in if you are away.  Change your computer's settings to automatically lock after 5 minutes of non-use.  Make a point to completely logout of any secure website and close the browser window every time you complete a job.  Failure to close the browser window may result in the login session remaining open.

System maintenance :

Maintain your systems with the most current security patches (aka Service Packs) from your system's vendor.  This will close most of the known security holes.  Limit the amount of services and applications you are running simultaneously.  The more services running, the greater the risk of exposing the system to exploitation.  Finally, remove any programs or applications not being used and defragment your hard drive on a regular basis to help your system's overall performance.

Password Security :

Never store password as plain text or write down on paper. Use encryption utilities if you have to store the password in a file for some reason.  Do not share your password with your coworkers.

Payment Card Industry Compliance :

The major credit card providers developed several standards they suggest anyone accepting credit cards should abide by.  They include the following:

1. Never store sensitive credit card information, especially the full credit card number, after a transaction is complete.
2. If any data pertaining to the card holder must be kept, protect the stored data.
3. Require a unique username and password for anyone using the system.