Yeah, I actually work as a cashier at Pacsun.
All you need is a really really strong magnet.
Oh wait.... Oh crap, thats not the magnetic kind. That is the kind you have to have a special "gun" to take it off with.
I'm sure you can just go in there and tell them the situation. They won't be assholes about it, especially since it was THEIR fault they didn't take it off when ringing you up.
I'm just curious as to why the alarm didn't sound and it was checked when you were leaving the store....
Rox's wiki looks like it should work, but the biggest thing I recommend is to go in and tell them the situation.
But when you enter the store, walk STRAIGHT to the cashier so they don't think you just picked it up. And if you can, get in their face about how it is their fault they left it on. I hate it when customers are assholes to me, but you HAVE TO listen to every word they say
Haha, awww... I'm causing a bad day for a fellow retail sales assistant... Shucks