I have a Non.Imm.B multi-entry visa and a WP. For various reasons, I don't have an extension of stay so I have to leave every 90 days. That's cool but I'm sure I read somewhere that I have to do something or fill out some form to keep my WP active, otherwise it gets cancelled if I leave the country. But I can't find anything like this here or anywhere else on the net.

Do I need to do something before I leave or have I just mis-remembered?

Your WP remains valid when leaving the country, as long as you return on the correct visa (which you have).

Your WP remains valid when leaving the country, as long as you return on the correct visa (which you have).

And shouldn't you return before the WP expires?

The OP obviously has no intnetion of letting his work permit expire and therefore the question of returning it does not enter into this discussion.