When I had my S2 I bought Swiftkey for the keyboard. Now I upgraded to the S3 I downloaded it again but it will not let me keep it as default. If I restart the phone it defaults back to the Samsung keyboard.
Go into Settings - application manager - All. Scroll down the the Samsung keyboard and press on it. Then press on Force Stop. That should stop the default keyboard from defaulting away from Swiftkey, at least until you restart the phone.