I'm not bilingual so you don't want to hear from me, strictly speaking....but I couldn't resist commenting!
As a native English man I obviously know English but I have been learning Greek for many years due to having a Greek wife and I can hold a very basic conversation. I was actually told be a Greek visitor to my workplace that, after having a brief but reasonably successful conversation with them, that my level is like a Greek infant !
Main problem for me/us is my wife can speak English perfectly so we hardly ever try to speak in Greek.
However, I very recently read a book 'fluent forever' which has lots of good tips, I think, on how to learn a language efficiently.
I was trying to incorporate them into my daily life when our little boy came along and everything time-wise has gone 'out the window'!
So finding time to practise guitar, let alone Greek (which is lower down the list), is always a challenge!
Too many times when I have an opening (when everyone else has gone to bed) I find myself getting drawn into delcamp threads...like this one!!!
However, the ideas the book (and site) outline are things like the using flashcards with spaced repetition methods (Anki is a free tool that does this) along with targeting the most used words in a language 1st as well as a few other good ideas.
The spaced repetition idea (which I think someone mentioned earlier in this thread) is really the big thing, for me, which could/should help someone to learn a language. The basic idea being that if you test yourself on the things that you are just about to forget or have just forgot, then you will get them into long term memory by testing your recall when this happens. But like everything, you still need to put time aside to get started (my problem at the moment - setting the flashcards up in anki...) and then continuing to use the method.
Interestingly, the subject of whether my wife should speak Greek to our child whilst he is young has come up. She thought that learning Greek might not be very useful seeing as we live in England but I have heard many times how children pick up languages much more easily than adults so I've hopefully persuaded her that she should try so we can give him at least 2 languages and be, hopefully, bi-lingual-ish when he's older.
I did French at school and my wife did German so if we dust off our school books then maybe we can get our little one up to 4 languages!