There is no way to trick the iPhone fingerprint. This is the same for the retina scan on the newer phones. When you set up the phone and you select to use the fingerprint the phone asks you to test the print 6 different times. You'll need to place the thumb on the button in different ways for the scanner to scan the thumbprint image and store it on the phone.
Each time your phone is shut off the thumbprint won't work. You'll need to enter your 6 digit code to the device in order to open it up for the first time. After this, you can start to use the scan thumbprint image again to open your device. The only other alternative you have on this is to wipe the phone thumbprint and start all over again.
If this is your phone this shouldn't be an issue. However, if you have purchased this phone online or have found this phone then you'll have a problem getting this phone unlocked so you can use it. There are several new security features on the phone that will block you out and not allow you to open the device unless you know the email address and password for the device.