As you've mentioned - if you don't use a skill, you lose it. Couple of suggestions:
- Do a little bit every day, or at least every week (no longer than that).
- Write a list of things you want to practice with JS and tick one off each time. Doesn't have to be huge, it can be as simple as practicing using a single method.
- I keep a js-notes file where I write notes on various features of the language I'm learning. I describe them in my own semi-random ways because that helps me find the relevant notes later.
- I keep the little files I made to play with different bits of JS. This forms a rough code library, so when I'm in a hurry I can quickly grab the syntax for something I haven't done for a while.
- Build something. Having a project drives most people more than an abstract thought of "I should learn this". Make a To Do list, build a game like tictactoe, do classic programming drills like fizzbuzz or a fibbonacci generator. Something like the Odin Project might help here - theodinproject.com/javascript-and-jquery (some people at work ran beginner JS workshops based on the Odin Project and it worked pretty well).
Hope that helps :)
Worth noting this approach is not at all JS-specific, it's how I suggest people learn all new languages and tools.