I'll cover the obvious culprits:
- Turn off ALL controllers including any Guitar Hero, etc that you may have.
If you have any non-Sony ones, even try removing batteries. Some controllers try to forge a Bluetooth link, even when you think they are off.
- Put PS3 into off mode (red light)
- Start using PS button on controller

That normally works.

If it doesn't, read the following thread. Someone else had a similar problem, and there's a solution that worked for them:
http://www.littlebigplanetguide.com/thread/3683827/2+sackboys+for+1+controller+from+the+startup+plz+help

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