[Brian]

Happy to help, Jochen! The "Archive" command under the File menu in OmniFocus for Mac lets you define the date you want to archive before.

Just pick 1/1/2013 and any items you've completed or dropped will be moved to the archive database.

After archiving, if you want to move any uncompleted items as well, you can do so with the following steps:

1) Select Perspectives -> All Items from the menu bar.
2) Select File -> Open Archive from the menu bar.

Next, click on the "Library" item in the sidebar of the window showing your database (not the archive, in other words.) Just do a "Select All" in the sidebar of that window, then drag everything over to the archive to copy the information there.

Now that you've copied the existing items to your Archive, you can delete them from the main OmniFocus window and start over.

Hope that helps!