The key to being fluent in a language is to think in that language. Easier said than done! To a certain extent you can prepare by learning a lot of general words like "thing", "stuff", "gizmo", "person", "machine", and so on. But mainly, there's a technique to limiting yourself to the words in the new language that you do know. I ran across a wonderful explanation, How To Think in English by Rachel Smith.
Of course her explanation is aimed at ESL students but the technique and exercises work for any language.
I've used these exercises over the years, myself, but Rachel came up with an especially good suggestion I never thought of. Try this just as you are dropping off to sleep. You might even dream in the second language!