It's actually really simple - instead of spending an entire day thinking about what project to start working on once the old project is finished, you can just spend a few minutes every day thinking about what you could work on once you'll finish the work on your current project.
One of the most annoying thing about us, humans, is that we love to procrastinate, and we love to leave everything for the last moment. When it comes to project ideas, we love to not think about future projects until we finish the one we're currently working on.
And I kinda get why. Most people don't want to start working on too many things at the same time and never finish any of them. However, no one said you should work on that secondary project before finishing the first one. I'm only talking about thinking about future projects.
You see, our brains are jerks sometimes. Once in a while, we'll desperately be in need of an idea, of something to work on, of a solution, and our brain will do whatever it can to not think of one. But, when everything's fine and we don't need anything from our brain, that's when it decides to show its true power and think of dozens of ideas in just a few minutes, especially if we can't write them down.
Take advantage of those random moments when your brain decides to prove to you what power it has, and write down the ideas you get. You may not work on them now, or anytime soon, but when you'll finally decide to work on something, you'll have those projects that you thought about a long time ago.
Basically, the easiest way to always have projects to work on is to think about cool future projects while still working on something at that present moment. Once you get an idea, keep it, and work on it when possible. Don't wait until the last moment, when you'll desperately need a really cool idea, because chances are that you won't get one at that moment.
hiveblocks