Splitting A Library
If you have a library that has gotten too large and you want to split it into multiple, smaller libraries, there are a couple different ways you can go about doing so.
First, click the "New Library" button and choose the location you want to create the second library.
Then, if you are a registered user, simply drag one or more albums of photos from the album list of your big library onto your newly created library. This will copy those albums over to the new library, including all their dates, ratings, comments, etc.
If you have not registered yet, you can instead create a folder on your desktop, open up your big library in iPhoto and drag out the pictures you want from the iPhoto window into the folder you created. Then, use iPhoto Library Manager to open the newly created library, and finally drag the folder from your desktop into the iPhoto window, which will import those photos into your new library. Using this method will require recreating your albums and other photo attributes manually in the new library.
After you have copied the photos to your new library, you can remove them from the big library, and you will have successfully split your library into two.
Duplicate & Prune
Use the "Duplicate Library" command in the File menu to make a copy of the library folder that you want to split. You can then open one copy of the library and move whatever photos and albums you don't want in that library to its trash. Then open the second library and move the rest of the photos into that library's trash. After double checking to make sure that no photos have been trashed from both libraries, empty the trash of each library.