1. Reboot by holding down the top button until the "Slide to power off" message appears.
2. Slide to power off.
3. Power on by holding down the top button again.
4. As soon as the Apple icon appears, hold down the volume up button. Hold until phone has fully rebooted.
5. At this point you will be in safe mode and you can go to Cydia and remove the offending package. Note: All Cydia packages should be inactive at this point, though we've seen a few that continue to load anyway.
6. The next time you reboot without holding down the volume up button, you will be out of safe mode and your Cydia packages will load.