How do you avoid forgetting a language?

Question for the polyglots out there. How do you avoid forgetting a language?

I speak Portuguese (N) and English (C2) and find it pretty easy to navigate through these two languages. I also speak some French (B1) and have been living in Italy for the past 6 months, which puts me in daily contact with the Italian language (became roughly A2-B1). I have no one to practice French with and I feel like I have been forgetting it. I intend to eventually move to another country and I wouldn’t like to forget Italian as well.

What is your tip to not completely forget a language even though I have no one to practice with? Also, how can you do that without mixing languages up? (Sometimes when I try to speak French I notice I end up mixing it with Italian, or when I try to remember sentences in Russian I end up saying them in German, two other languages that I’ve attempted learning before).