Two requisites, instruction by native speakers. And almost complete immersion from day one. Both are completely lacking in how foreign languages are taught here. Drumming grammar and teaching people to translate on paper and in their heads is one reason most of us are monolingual, while lots of Europeans (Brits excepted) tend to be conversant in at least two languages.
Past the age of about six learning any new language isn't easy. There are sounds in Arabic that require one to do "unnatural" contortions at the back of your throat. But it opens the person to new worlds and ways of looking at the world.