My Top 8 Duolingo Tips
Use the "practice" mode (the barbell button) about as often as you complete new skills. Duo will not make you repeat the material enough. This leads to poor retention, and worse, you may not even be aware. A healthy portion of mistakes in repetition are actually a good sign, because it means you are not slowing down your progress too much.
Go with the flow. It is natural and healthy to have episodes of interest in a particular language (for me usually a few weeks at a time) and then you are more interested in another language or something else.
Working on more than one language at a time is generally a good idea. It helps you manage your motivation level and curiosity. It forces you to encode the material more deeply than required to just pass the exercises. Keeping multiple languages "straight" is a wonderful way to exercise your brain.
It pains me to admit it, but sometimes there are things Duo doesn't teach you well enough. For example the Chinese course doesn't explain Chinese characters enough, in my opinion, and I can see why. Try to find a book or other resource on how to write, interpret and remember them, maybe something about mnemonics. In other languages you may benefit from a good resource on grammar. Duo does teach grammar, but it's not designed as a grammar reference. Memorizing specific cases until you discover the rules on your own may not be the best use of your time and frustration tolerance.
Try learning a language from a foreign language. For example any "For English Speakers" courses. But there are also courses unique to Spanish (Catalan, Guarani). It won't fry your brain (too much), but you'll grow. However, the "for English" courses usually have the highest quality content in my opinion.
At some point, probably after earning the golden owl, it's time to start reading texts outside Duolingo. It's the best and fastest way to repeat, increase and refine your vocabulary and grammar. Many nuances are only discernible through living language. And in contrast to video/audio material or live speakers, you can consume it at your own pace. Also don't look up every known word. Try to build an intuition which words are important or guess the meaning from the context. If possible, switch to mono-lingual dictionaries. Maybe pick a novel you have read in your native language.
Pay attention to the pronunciation. Foreign languages often have a different set of phonetic components, including quite different vocal tract movements. They often have a different intonation and timing patterns. Paying attention to those details isn't easy, because your native language can make you "blind" for those distinctions, but it will drastically improve the impression native speakers have of your skills. But don't be ashamed of your accent. An accent isn't really an impediment, and some people find accents interesting or even sexy.
Many people already do this, but in case you're not: Picture the foreign word with a similarly sounding word in your native language, even if those words mean nothing similar. Then associate the actual meaning with that word. It's one of the easiest mnemonic techniques, but also very effective.
Thank you for your post.
“ ... a different intonation and timing patterns. Paying attention to those details isn't easy, because your native language can make you "blind" for those distinctions ...”
That bit was so true in my case. I could type in my answers correctly in Greek. But when speaking in, Duo could not understand what I said. I had to practice my pronunciation for a long time, often with great frustration. And shouting at the screen “that’s not what I said” does not help ha, ha :) But persistence pays off, and now, happy days, both Duo and google translate understand my Greek speech :)
You can often find detailed descriptions how the pronounciation works biomechanically, like where in the mouth the sound forms. It's tedious and complicated to understand those nuances, but it pays off.
Often these preliminary details are omitted, because they are not sexy, or even teachers themselves have trouble with them. It doesn't help you to say hello or ask for the bathroom. But it's even more frustrating to learn this stuff by trial and error.
Our (German) English teachers had to fight us the whole eight years on the difference between the and s.
In addition to daily practice with Duo, I take Italian classes and also practice with friends in the class. This provides lots of practice actually speaking the language as well as learning grammar rules and reading stories in Italian. But I have to say my consistent practice with Duo has given me an advantage over my fellow students who do not.
In my opinion presence-classes are too slow and inefficient. Maybe because teachers are more expensive than computer time. Or that the class has to cater to students with different speeds.
The best use of a teacher's time may be to fine-tune pronounciation or to provide more direct and continuous motivation.