Activating a PlayStation 3
Switch on the PlayStation 3 you want to activate and navigate to the Sign In option in the PlayStation Network menu using the XrossMediaBar. Sign in to your PlayStation Network account, then select the "Account Management" option. Navigate to the System Activation option, then select "PS3 System." Select either "Game" or "Video," depending on which type of content you want to use on that PlayStation 3, then select "Activate System."
Deactivating a PlayStation 3
Switch on the PlayStation 3 you want to deactivate and sign in to your PlayStation Network account. Select the "Account Management" option, navigate to the "System Activation" option and select "PS3 System." Select either "Game" or "Video," depending on which type of content you want to deactivate on that PlayStation 3, then select "Deactivate System."
Restrictions
You can only activate a maximum of five game systems on a single PlayStation Network account, counting both PlayStation 3 and PlayStation Portable. Additionally, according to the PlayStation Network terms of service, you may only activate game systems you own or on which you regularly play on your PlayStation Network account, as sharing games and other content you have purchased with others is not the intended use of this feature and is against the terms of service.
Considerations
Although the five game-system limit may seem generous, you can quickly run out of allowed activations if you aren't careful. For example, if you send your PlayStation 3 in for repairs and you receive a refurbished PlayStation 3 in return, the old PlayStation 3 still counts as one of your activations and you will be unable to deactivate it since it's no longer in your possession. Because of this, you should always deactivate a PlayStation 3 before sending it in for repairs, as well as before selling it or giving it to a friend or family member.