Use memrise.com to learn vocabulary
It's a pretty neat way if you want to focus on vocabulary.
I really like this site, it does a better job at memorizing vocabulary but it doesn't touch Duolingo's methodology at learning a language. Actually I think it would be almost impossible to start learning a language from scratch just from memrise.com. Either way, more quality resources for language learning is never a bad thing. Thanks for the mention.
@lingua Oh it wasn't my suggestion. I have my doubts about flash cards, and I'm not the only one. Here's a polyglot who threw his flash cards away: https://theczechexperiment.wordpress.com/2011/11/04/flashcards-vs-extensive-reading/