In a safe place where you will remember where they are when the worst happens to your Palm.  You should store your downloaded applications in a centralized location for easy access as well in case you have to reinstall your applications.  I recommend that every time you purchase an application, you store the application’s registration code in a document somewhere and print the sheet out periodically.  That way if something really really bad happens you will have a hard copy.  I have found a neat tool called RegTracker offered via PalmGear that allows you to store your registration codes and application information for both Palm and PC applications. 

PalmGear does save all purchased software in your My PalmGear account for you to reference at any time.  You can find or request all your registration codes as well.  Simply click here to set up or log in to My PalmGear