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I have used this many times and am now currently on my 3rd free month of Plus! Make sure to turn off automatic renewal.http://psplus.software.eu.playstation.com/
Don't forget to cancel the auto subscription if you don't want to pay for it after the trial period.
Go to the network icon on XMB
click account managment
go to transaction managment
go to Services list
click on your package
then click cancel automatic renewal
Simple as that . there might be an easier way-- but thats how i do it.
P.S. if you dont turn it off-- they will use the money in your Playstation account / or if you dont have money or not enough they will take the money from your card on file