Lots of people mentioning tricks and "hacks" that work for them, and seconding techniques from the article, which is helpful.

But after reading a lot of these sorts of articles and threads over the years, the most consistently useful advice that I've seen is simple:

Learn discipline, because motivation will always fade.

It's like getting yourself to exercise regularly, or read at least a half-hour every day. Easy to get started, but even easier to skip a couple of days, then a week, then to stop altogether.

Like they say at Nike: "Just do it."