There were many posts like this one on the Duolingo forum, and I thought I'd make one here too.
If you who are reading this, and have any tips you would like me to add to this post, you can PM me about that here
For any learners who are new to Duolingo, I hope these tips will help you!
- Read the Tips
(Every course on Duolingo does not have tips, but the majority of them do.)
The Tips on duolingo are unavailable on the app, but you definitely don't want to miss them. The Tips help you with sentence structure, and well as pronunciation.
- Read the sentence discussions.
I think both the sentence discussions, and the Tips are things you can easily skip over, but they are both very helpful features!
As of now, you are unable to comment on the Duolingo sentence discussions, so, this forum is trying to make up for that loss, you can read more about that here and here.
Even though the Duolingo sentence discussions are locked, do not fear! Possibly, the question you have about a sentence will have already been answered, I have found that there are usually also many tips and helpful pointers from native speakers.
- It's okay to use a translator sometimes
Using a translator too much can lead to you not learning, and you not using your brain, the translator being the one doing all the work. But translators can be very helpful, if you use them with discretion. They can also be very helpful with spelling.
- Take notes
Taking notes while you learn is very good for your memory! Linking your hands to your brain can help you remember things better.
- The hover method
It’s been said that the hover method on Duolingo is the best way to recall and remember words and phrases. You can learn all about it here.
Tips from other users
- Type your responses
If typing your response is not possible, try to formulate your response first, then use the word bank to get your answer. (duokeyboard is an awesome extension for Chrome if you are on the web).
Thank you to [mention]mathemajician[/mention].
You can access the web version from a browser on your phone, you don't have to use the app.
Making flashcards or using Anki or Memrise is very helpful.
It is worth mentioning that there are shared Anki decks/memrise courses built to go with Duo specifically, search for Duolingo + language. I owe whoever built the Memrise Russian one a huge thank you 
Thank you to [mention]Corinnebelle[/mention] and [mention]mathemajician[/mention].
- You can also use other methods to learn your desired language as well
Duolingo is very good at one job only: drilling grammar and vocabulary. However, speaking a language is so much more than just learning to translate properly. As part of your learning process you have to learn to construct your own sentences, rather than just repeat like a parrot. Duolingo cannot help you with that creative aspect of language-learning.
That means it should not be used as the sole language-learning method, but rather as one tool in a language-learning toolbox. And of course the "learn language in 15 minutes per day" slogan is false advertisement (kinda like saying "develop your muscles by doing 1 push-up per day" - nope, not gonna happen).
That doesn't mean that Duolingo is useless. It simply means language learners need to employ a diverse array of language-learning tools, and Duolingo can be one of these tools for beginner-intermediate level learners. Do a few lessons on Duolingo, then maybe watch a movie in your target language, read an online newspaper in your target language, and then come to Duome to try to participate in a discussion in your target language. 
Thank you [mention]wayfarer[/mention] and [mention]Stasia[/mention].
- Join Duolingo events.
After more than a year of Duolingo I couldn't even say one sentence in Portuguese, and I wasn't even aware of that until I started participating in the events. My progress since then has been very fast.
Thank you to [mention]EranBarLev[/mention].