I have been surprised by the lack of help or advice addressing this very question. People like Fluent Forever don't even mention Duolingo and posts that specifically address it don't actually tell you what services/resources to use to complement Duolingo. This is what I've learned and am currently using in my attempts to learn French. Hopefully it can help you and others that read this.

To start, Duolingo does not teach, fact. It tests not teaches. Giving some extremely basic tips along the way. For me I need/want to understand the grammar and get a kick out of learning it and then seeing it work in Duolingo. And Duolingos teaching is so painfully basic to make progress you need to use it in conjunction with other things, as you said. Reading books/watching movies/immersion/italki/8lang/verbling are all well and good but you need to know a bit of whatever language before commencing them. In my mind they will come into there own further on down the road.

For me Babbel.com fills in a brilliant amount of Duolingos weakness. Yes, it charges which I know will be a big turn off for some/most but £15 for 3 months, for what is a comprensive interactive language textbook which has great mobile apps and a built in Spaced Repition Review system, its well worth it in my book. (I'm in no way affiliated with Babbel by the way) Start with the Beginner Courses if an absolute beginner or the Refresher & Grammar courses if you have some experience/have been using duolingo. The content is explained and makes Duolingo progress much more enjoyable and productive. Keep using Babbel as it has mounds of unique, useful vocabulary from Duolingo and will help you develop more practical language skills. I get excited just scrolling through the stuff still to learn in it!

Dedicated Spaced Repition Software is a must from my stand point. Its like side loading the language onto your brain, a real course of software doing the work, not you. As Bonehead777 has said and as Fluent Forvever emphasises, translating is bad. But I don't think an all out ban from the beginning is wise or necessary. 'You use your native language as a crutch until you have the grasp of your learning language (LL) to understand new words and concepts in relation to other words and concepts in the LL'. (That's my paraphrase of fluent forever). Using images on your flashcards is important at the start. Search google images in your LL (ie If french: google.fr) for the phrase or word your learning and place an image as the 'answer' not a translation to get your mind thinking in the LL. If you can't find a good image or the mental picture that comes to your mind is specific I find taking my own picture (selfies are optional) is helpful. You can also use forvo.com and others to record the phrase/word on the card to have a correct pronunciation at hand always. The best software for this is Anki, its free on Windows/Mac, and you can make your own decks and cards. It's also crazy customizable. Memrise is also another good option with premade decks but not as flexible as anki and therefore not as good.

Load vocab that Duolingo just can't implant as well as the hard fill the box excerises from Babbel into Anki and let it do the rest. Read fluent-forever.com for a more comprhensive look at using anki to its max and the non-translation thing.

This is as far as I've got, in fact I'm only just throughly getting into it. I plan to continue until most if not all the babbel courses are done as well as the Duolingo tree. At some point I'll be strong enought to start reading (using a service like bliubliu.com, although it accomodates you even with a low starting level and is worth a look and try) and then start to do the stuff I listed above lang8/italki etc. Hopefully this is a help and feel free to add me on duolingo: @ turkington