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This has been tested on a 60GB PS3 on 3.55 Team-Blizzard Custom Firmware, spoofed to 3.56. With CO

Black OPS FIRST STRIKE
I will show you how to game share, and keep your activations.
Firstly game share how you normally would and activate the content you want.
After you have activated the content, enable multiman (or your favorite backup/file manager) and go to:
/dev_hdd0/home/[USER WITH CONTENT]/exdata/act.dat
Copy this file to a pen drive or somewhere on the PS3.
Now deactivate the account via Account Management > Account Activation > Game > Deactivate.
If you play your game now the content will not be there (for black ops it wasn't there).
Now go back to multiman ( or your favorite backup/file manager) and copy the act.dat file back to:
/dev_hdd0/home/[USER WITH CONTENT]/exdata/
Start up your game and the content should be working, without wasting an activation!
Do not delete the user account on the ps3 with the sign in details as this will disable the content. But you can deactivate it!
Will test this further!
EDIT:
More detailed/easyer to read tutorial:
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