I just finished up the Fluent Forever book by Gabriel Wyner and thought it was great. I'm interested in learning Spanish, and I like the approach that the book takes. It has solid advice and is backed up with some research citations.
In this post I'll break down my key takeaways from the book and the accompanying website which has rich videos. Check out Derek Sivers's post for an in-depth review.
Spaced repetition is key
Consistent spaced repetition underpins the entire system. I won't go into heavy detail in this post, but it is basically a way to create flash cards that you see at the optimal time for retention. An algorithm determines how often you will see cards, so you only review cards that you actually need to review. By setting up small cards that you can review, you can learn and retain a lot of information.