If you've unlocked with SAM but haven't backed up your unlock ticket, you should do so immediately by following this tutorial. To follow these instructions you will need to have the unlock ticket and iFile installed. You can get iFile from Cydia.
Insert your previously unlocked SIM card and make sure SAM is disabled in SAMPrefs.
Email yourself the zip archive created when you backed up your unlock ticket. Open the email on your iPhone and tap to open the attachment. When prompted to select an application to open the attachment with choose iFile.
When iFile opens it will ask you to choose between Zip Viewer and Unarchiver. Choose Unarchiver and tap the Done button when it's completed extracting.
Tap the Edit button at the top right of the screen and select the Lockdown folder that was just extracted.
Press the Clipboard button at the bottom right and choose Copy/Link from the popup menu. Then press the Done button at the top right of the screen.
Use the back button at the top left of the screen to navigate to the /var/root/Library folder. Press the Edit button at the top right of the screen and select the Lockdown folder.
Tap the Trash icon at the bottom of the screen and choose Move File to Trash from the menu that appears.
Tap the Clipboard icon at the bottom right of the screen and choose Paste from the menu that appears.
Press the Done button at the top right of the screen then reboot your device.
Once your iPhone has been restarted it will pick up the network for the unlocked SIM card.