I used three paper clips. To hold an attachment together, to hold the N-400 application, copy of green card with the pictures together and the third to hold the check and the two packages together.
I read on many web sites that the BCIS doesn't like stapled applications or folded applications. Therefore, I made sure that my application is easy to be handled by a BCIS clerk.
I did the same with my Green card application, several years ago.