The best way to learn a language is to be 2 or 3 years old and just absorb it.
There are memory techniques for learning languages but, really, if you lack a child's memory ability, then I do not see how you can learn a language rapidly without...
Already knowing a similar language. Romance languages seem to be fast-to-learn for people already fluent in a Romance language. Or reading Japanese characters might come quicker to a Chinese student than to a Western student.
This is a really frustrating conclusion. When I was younger, I believed the foreign language learning lessons of the many memory books I was reading; but, having spent a lot of time just trying to learn the basics of some European languages, I see those techniques as being helpful on occasion but of limited application. It's best to be a young child; otherwise, you have a lot of work to do.
I think that, if a vocabulary word is resisting your attempt to memorise it then a memory technique is good. But language communication takes place at such speed that there is no substitute for rote learning language.
I do like the idea of gently learning the first 100 words of a language by playful memory techniques. If you think the Mandarin word for China sounds a bit like John Wayne, and that helps you to remember it then great!
I also think that, if you are given time to pause, and you have a memory technique for recalling a word which you want to employ, then a memory technique would be good then too.