Although you're afforded some bit of copyright protection when you press the shutter, to really protect your work, you must register your images with the US Copyright Office. Carolyn Wright has provided a step-by-step guide at naturescapes.net and Aperture can help make the process easier.

Make a New Button Set
I found it useful to make a button set called Copyright (Figure 1).

1. To make this button set, make sure that the control bar and keyword controls are visible (the commands to show them are under the Window menu).
2. On the popup at the bottom right, select Edit Buttons.
3. Under Keywords Library, click the Add Keyword button and add three keywords, "registered," "published," and "do_not_register."
4. Click the Add new button set button on the bottom left of the window and name the new set "Copyright." Make sure this set is selected, and the Contents table should be empty
5. Drag and drop the three new keywords (use the filter field to find them if you already have a lot of keywords) to the middle column.
6. Click OK when finished, and select the Copyright set from the popup

Find and Export Published Images

1. If you've never registered an image but have published it, select the image(s) in your library and click the published keyword button.
2. Make a new smart album (File > New Smart > Album) and set it to show any image whose keywords contains published
3. If you have NOT already registered some published images in this group, skip to step XXX. From that smart album, select all images and select File > New from Selection > Album.
4. In that new, non-smart album, find all images with the "registered" keyword (or, if you haven't added that keyword, select the images that you've already published and registered, click the registered keyword button, and then filter the images).
5. Select all the registered images and delete them from this album.
6. Clear the filter. The remaining images are published but not registered.
7. Select all images in this album and export a version of them at whatever size you prefer (I would suggest JPEG - Fit Within 1024 x 1024) to a new folder.
8. Choose File > Print and make a contact sheet using the Name Only metadata set. Save it as a PDF, too, to the folder with the exported images by clicking Save as PDF from the print dialog.
9. Select all images again and click the registered keyword button.

Find and Export Non-Published Images
The way I rate images is that three stars and above are potentials for publication and submission, and I'm writing these steps with that idea in mind.

1. For each project, (I do this per-project to be able to create smaller groups of images more readily and to provide some structure to large submissions to the copyright office), filter the browser to only show images rated three stars or higher.
2. Select all of those images and choose File > New from Selection > Album.
3. In that new album, filter for the "registered" keyword and delete any registered images from the album (select the images and press the Delete key).
4. Clear the filter and scroll through the album. If there are any images you don't wish to register (e.g. they're personal photos that you want to keep and backup but not register), select them and click the do_not_register keyword.
5. Filter the album for do_not_register images, select them, and delete them from the album.
6. Clear the filter.
7. Select all images in this album and export a version of them at whatever size you prefer (I would suggest JPEG - Fit Within 1024 x 1024) to a new folder.
8. Choose File > Print and make a contact sheet using the Name Only metadata set. Save it as a PDF, too, to the folder with the exported images by clicking Save as PDF from the print dialog.
9. Select all images again and click the registered keyword button.
10. Repeat for each remaining project.

Once you have your images together, burn them to a CD, print the contact sheets if needed, fill out the appropriate form, and send it into the Copyright office.