You can organize the references in your library by putting them in groups. Groups make it easier to export specific formatted references, can make it easier to find the reference you're looking for, and make it possible to share a set of references with a collaborator.
To create groups, go to Organize->Manage My Groups
Once your groups are created, they'll show up on the left column of the My References page.
Once you've created a group, you'll want to add references to it. Check the boxes to the left of the references of interest and then choode "Add to group..."