If you installed apps or tweaks from Cydia it's probable that one of them is causing the issue (like Activator) as it has not been tested and edited to support iOS 6. Also, the iOS 6 jailbrek is tethered, so you must connect your iPhone to redsn0w and click on extras>just boot in order for it to boot properly, you must do this every time you want to turn off or reboot your device.

There's no way to do that... yet