There are a million websites, books, pinterest posts, and online classes available that offer writing exercises. I think I've used most of them, at least a little. They can be fun, and stretch your creative muscles before a long morning of putting words to paper (or screen, most likely), but not all are equally useful. I've noticed that some are more helpful than others, so here's a few things I've learned about what's available.
The most important thing, of course, is what you put into any exercise. Doing it to say you've done it, no matter how brilliant the idea, will not teach you much. If you find it difficult, then scribble down whatever to fill a word count, you've wasted your time. It's not a rough draft, where the goal is just to get down your ideas so that you can find their flaws and correct them; an exercise is complete in itself. For the most part, you're focusing on some small aspect of craft, so you need to pay attention to that. When you've finished, evaluate your work. If you have someone patient around to read it, ask them. It's almost impossible to see your own work clearly. In this way, you get the most out of your work.
Choose exercises that focus on your weaknesses. I struggle, always, with tight, coherent plotting. So I choose exercises that work toward plot outlines, or character arcs. It's fine to pick something that is easy for you, if you just want a little boost. But the reality is, progress is only achieved through pain. You must do what is difficult in order to improve your skill.
You still need to do the work. An exercise or two to stretch your brain is fine if it helps you warm up, or approach an issue in your work. But if you spend your writing time fiddling with practice exercises, you're not finishing your work in progress. The only way to get better at writing is to write, evaluate, and write again. The more works you finish, the more you will know about finishing. Each moment of writing IS practice.
So find some good exercises for 'deliberate practice' where you need it, and see where it takes you. Just keep in mind that the most deliberate practice is the completion of your work in progress.