The best way is immersion (roommates, television, taking regular college classes in another languages, etc) but if that's not something you can do, the next bes

The best way is immersion (roommates, television, taking regular college classes in another languages, etc) but if that's not something you can do, the next best thing is to get gigs translating documents. Underbid professional interpreters and be upfront about the time it will take. If possible, start with technical documents such as electronics manuals - they require minimum grammar and vocabulary. It worked for me to get started on English... when you have a paid gig with a deadline, you are much more motivated.