IPhone 3g help please?

I have an iPhone 3g.. its updated to 4.0.2, but I want to downgrade it back to 3.1.3 because I dont like the 4 upgrade. I downloaded the firmware from felix bruns and when i plug it up to itunes and try to downgrade it by using the firmware, it extacts all my software, but it doesnt work, it says error, phone could not be restored.. what do I do?