The only suggestion I have is practice, practice, practice, if possible with native speakers.
English is my first language, German my second (which I learned from my grandmother), Spanish my third (I spent several years of my childhood in Ecuador, at which time I was fluent, but rusty now).
One trick I found useful, if there is no one to converse with, is to ask yourself how you would say something in the language you wish to learn as you go through your day's activities. For example, "Oh, what beautiful weather we're having today." or "I'm walking down the hall, but I can't seem to find the office I'm looking for.", etc.