I haven't seen this mentioned often, but considering how many people are surprised when I tell them, I figured I'd share these tips here.
If you plan to travel with your SBR, you can download the form from the ATF website, fill out the PDF, print it out, add your signature, scan the document, and fax or email it. The addresses are on the instructions included with the form. You can have up to 3 firearms per form, and you can request up to a whole year.
I just did my first one for two SBRs. It took just over a week for the actual approval, and two weeks to make it's way back to my mailbox. I requested a whole year since I'll be taking these two to matches just over the border in NC.