After learning English, I studied Finnish for some years and I attained a decent level. When I was studying Finnish, I noticed already that it was sort of "displacing" my German (A language I had only "half-learnt", I could more or less read it but I never was conversational), and even though the two languages are not related at all, whenever I tried to speak German, Finnish would keep on coming to my mind, and all the little words, prepositions, numbers, pronouns, etc, came to me in Finnish. I didn't care too much, for I wasn't using German, and I just let it be.
However, now, I have been studying Czech for some months already and, although my level is still clearly very low, I can tell that it has begun to displace my Finnish. I have not spoken Finnish in a long time, but it is a language I love and I try to read or listen to some Finnish every now and then, and I would definitely love to speak it again when I get the chance (when I travel there, for instance).
I have tried several things, like talking to myself and read just brief things in Finnish everyday but, since my focus is on Czech now, I still feel like it is going away. It is a very weird feeling and, interestingly, it happens with the most basic words: I am trying to say koska and I have to struggle not to say protože, I want to say mutta and what comes out is ale. And so on... Interestingly too, none of these languages have messed my English, which, even though it needs much improving, I am sure, it is at least quite stable.
What could I do to prevent that? Are there any known procedures to keep a previously learnt language when learning a new one? I mean, I guess a short answer could be just "hard work", just working hard on Finnish, but it is just sad to see that my Finnish gets worse now that I am studying a new language than if I wasn't doing anything at all, and regretfully I don't have as much time now to just commit myself to tough work in both languages.