One account only at a time
Below are instructions that Gunbuster posted the other day:
'''It's a little more complicated than that - you can only have one account active on the PSP at a time (i.e. you can only play US games until you make your UK account active, and vice versa), and simply logging into another account won't make that account active.
The easiest way I've found to do it is this:
1. Go to the Playstation Network section of the XMB.
2. Select the Account Management option and press Triangle, the select 'Delete Account' and press X.
3. Go to the Playstation store and login with your other account.
4. Download something small and free, like a theme or demo - at the point you do this, a screen will popup and ask you to confirm that you want to make this account active - select 'Yes' and all content linked to that account will become usable, everything else will become unusable.
If anyone knows a quicker way that doesn't require an internet connection, I'd love to hear it, as I could sell one of my Go consoles! '''
As you can see it's probably best to just use one account on there full stop