I really like the Assimil method of learning. I find it is more organic than Duolingo or flashcards. It makes you more proficient in conversation early than learning a whole lot of vocabulary.

My uni also offered a Dutch course with a similar style to Assimil. Loved learning it, although it is quite similar to German so it was actually super easy. No offence to you Dutch folk, but the way you pronounce your G is just terrible. xD