In short: immersion, SRS and cloze deletion. Screw textbooks, classes and any "this isn't proper material for a learner" elitism.
Learning a language takes 3000-10000 hours with the best techniques (length depending only on how closely related it is to one you already know), half that for decent basic fluency, about 2-4 weeks of intense practice for pub-level conversations. There's no free lunch, but it can be pretty tasty.
Techniques:
1) There is no Immersion like Immersion and Khatzumoto is its prophet. (Slightly kidding, but he's my favorite advocate of the approach and fun to read. And he is absolutely right.)
2) What's cloze deletion? Anki FAQ. Why does it matter? It gives you lots of context around unknown pieces, making them stick better. Also, it's fun.
3) Anki is the best SRS, see the site for an explanation how to use it. At first, you make cards "word -> translation". Then "easy sentence -> translation". Then "easy sentence with cloze-deleted gap" -> "full sentence". Try adding more context, like surrounding sentences in a conversation, audio and so on. Always go "target language -> translation" or "target language -> target language". (Contrasting with Khatz' advice, I'd recommend staying with translations and bilingual material for a long time until you can actually feel how sucky the translation is.)
4) If you like talking more than reading, copy Benny. Otherwise just consume as described.
This might seem a bit Japanese-centric because a) I study it and b) it has the best learning community evar, but this stuff applies to all languages equally. Some esoteric choices (say, dead languages) require some additional tricks to fix specific issues, but essentially it's all the same.
If someone'd like more details, especially for some specific problem, technique or language, just ask. I've been studying languages for about 4-5 years as a main hobby with differing intensity now and have tried pretty much everything that's out there in some form or another. But basically, there are no shortcuts. Do what's fun, imitate relentlessly, use an SRS so you don't forget everything again.
Since when has Japanese had the best learning community evar? It may be very friendly online, but in my face-to-face experiences public courses have fallen painfully short - I've been studying independently for only a year and a half and talk circles around AP students. Although they do still have an edge on me in such fields as "ordering meals in restaurants" and "presenting business cards", they really have no functional knowledge of the language at all.
Less Wrong is a large community of very smart people with a wide spectrum of expertise, and I think relatively little of that value has been tapped.
Like my post The Best Textbooks on Every Subject, this is meant to be a community-driven post. The first goal is to identify topics the Less Wrong community would like to read more about. The second goal is to encourage Less Wrongers to write on those topics. (Respecting, of course, the implicit and fuzzy guidelines for what should be posted to Less Wrong.)
One problem is that those with expertise on a subject don't necessarily feel competent to write a front-page post on it. If that's the case, please comment here explaining that you might be able to write one of the requested posts, but you'd like a writing collaborator. We'll try to find you one.
Rules
You may either:
or...
I will regularly update the list of suggested Less Wrong posts, ranking them in descending order of votes (like this).
The List So Far (updated 02/11/11)