ways to keep skill strength full for longer ?
Practice the words more, and be careful to get them consistently right. The equation for determining the decay rate is based off of number of right and wrong answers given for each word in a lesson, i.e. if you get 10 wrong answers, they figure you probably don't know the words all that well yet, and it will decay faster than a lesson where you've never answered wrong (although I think that will still decay, it will just do it much slower)
My German tree is golden almost every single day without exception. Here's how I did it.
I first went through all the lessons, didn't care about strength. I did learn a fair bit, though. Thereafter I only used the Strengthen button. I've been doing at least 100 XP a day, some days more. And I use the Android app, not the website.