Toying with a few strategies after the tree is done.
I started my Spanish tree in October 2013 and completed it in February 2014. It has not switched from all gold since a week after completing the tree. I then spent a 1000+ xp jaunt building skills and competing with myself on the app. Then I started the tree again. Well I've done the tree five times now, the last time through I went for a lingot on every lesson of every skill. Between each run through the tree I spent time in strengthening skills.
About a month or so ago, I tried the reverse tree and at first tries is was too simple, especially since I could test out of so much. So I played a bit more with the skill building on the original tree. Then a week or so ago I went back to the reverse tree. I shut off the English, so I don't have to type what I hear or repeat phrases aloud in English. I started from scratch and did each and every lesson for the skills I had tested out of. It took a while, but I started noticing that there was a distinct difficulty factor that was not apparent at first. I noticed that there were lot of pronouns attached to the infinitives in phrases involving items like tengo que inf. or voy a inf. and the same with other helping verbs. It did not show much at all in the original tree, but that's the way my friends speak in the real world, piling the pronouns on the end. Then in the various present tense drills, I noticed I needed to use the subjunctive very early on (e.g. I hope he wins - Espero que él gane) Plus there are also many phrases in Spanish that never came to light for me in the original tree.
So now I'm doing 80 xp on each tree each day; I need a life and can't keep up 250 per day. And I'm getting my immersion with life in general (TV, shopping, socializing with friends, cabbies and reading.)
The more I experiment with Duolingo the better I like it. Perhaps the next step is re-exploring the French I learned 50 years ago through a Spanish interface on Duolingo, because I'm not doing the English tree from Spanish 5 times.
I love hearing how others spend their online time with Duolingo, and the strategies they discover for themselves. Let's enjoy this!