Permanent notes are not intended just for the collection of ideas; instead, they help us develop ideas.
Therefore, it's important to be selective when creating permanent notes. Treating everything as thought it may be a permanent note can dilute the value of the zettelkasten as a whole. Instead, write them (as always) with an eye toward connection. Think about how they relate to your own thinking and writing. Let ideas emerge organically, but make explicit connections to other notes wherever possible.
One way to facilitate connection is by abstracting the ideas from their original context: this might help foster more points of connection between other ideas. Another strategy to help make notes more permanent is to comment on why you feel the note is important to your own work or thought.
Finally, write permanent notes as though they will be permanent. They should be written such that if you encounter one years from now, completely absent of any other context, the note should be completely understandable (including relevant citations!).