I've read some time ago (a bit before Christmas) that's it's nice to keep the tree in gold, plus I prefer it this way. I've never been so far in a DL course so I don't know if I'm doing something wrong but I spend all of time refreshing the lessons: I haven't done a single new lesson for one month, but practicing every day at least a little (10xp mini) because there were lessons somewhere else which decayed.
In fact, I spent so much time trying to keep the tree golden that I totally forgot the latest lessons I successfully finished and it's still decaying elsewhere.
What are your strategies? Maybe I should refresh the newest modules while hoping that they also strengthen the previous lessons?
It depends on how many lessons you have time for daily. If you can can't do more than 1-5 lessons/revisions a day, then you won't be able to keep all the skills golden as you progress. So far I've been doing more than 100XP every day, but now that I'm past the last checkpoint in my French tree, I'm struggling to keep it all neat and golden.
I agree that it's important to revise skills a lot to make them really stick... but if it completely stops you from progressing, then I'd advise you to focus more on new lessons and revise just one or two older skills every - or every second - day.
That being said, there are a few things that can help keeping your skills golden for a bit longer. Some languages have a Word section where you can find flashcards. If you do a couple of flashcard sessions (one session is for 15 words) that freshens up the most decayed words. For example, yesterday I had 4 decayed skills, but after doing some flashcard practice, 2 of those skills turned gilded again.
Strenghten Skills & Timed Practice helps, too. And sometimes when you revise newer skills, it affects older skills, too. (If they share some of the vocabulary, at least.)
I've finished my Esperanto tree and it's been golden for a while now. I didn't even realize that lessons could decay until I got to the last two rows of lessons and did them all pretty quickly over the course of a week. It might be different with other languages, but with the Esperanto tree I've found that if you take it slow and steady, revising extensively before moving on, the tree tends to stay golden of its own accord.
I have my daily XP goal set to 50 XP, and I think that much revision suffices to keep the tree from decaying. When I was working my way down the tree, on the days I was pushing forward I would do the lessons, and then do 50 XP worth of revision even if doing the lessons had brought me past my XP goal.
It's better to revise a lot and get as much locked in solidly into your brain as you can, than fly through the entire tree too quickly and not remember any of it.
I worked a lot per day to finish my eo tree quickly in 22 days. If you gain too little xp per day you may not be able offset the decay, that's what I realized. Then you may have to learn new skills without keeping everything gold. After finishing the tree however, like AislinC said, timed practice helps a lot. It actually completely solves it.
This may not be a great solution, but I like to "strengthen skills" to refresh myself and get in the mindset of the language. Then I do one new lesson. Then I finish with strengthening. This keeps things golden while slowly progressing so far. Maybe once I'm farther down the tree this strategy won't work as well?