If you’ve been infringed (and frankly, who wasn’t) there is a big difference on what you can do to the offending party, depending on your registration status. (Well, in the US, at least).
If your photo has been registered with the US copyright office you can claim statutory damages of up to $150,000 per infringed work + recover the attorney fees. If your photo is unregistered, you would have to prove damage, so there is certainly an upside for registering your photos with the copyright office.
One of the more common ways was sending a CD over, but this is quite a hussle. A new lightroom plugin – imagerights – aims at integrating copyright registration into your lightroom workflow. (And everything that is integrated into a workflow works better).
The idea is that you can select photos right inside lightroom and send them over to registration with the copyright office. Each batch is limited to 500MB, but you can send as many or as little photos as you want.