In reply to fire_munki:
The green ones (and orange, I think, but I've not tried those) are the most supportive, but might seem a bit too intrusive if you've got really flat feet - the blue ones might be a bit easier to get on with, in that case.
As above, though, proper orthotics might be what you need (though Superfeet I think still offer a two month trial period with full refund if you don't get on with them)