I'm not suggesting you turn into a planner. For some of us, the outlining thing is just never going to work. When I've imposed too many rules and goals on the project, I can't even write it anymore. At some point it stops being something I want to do and turns into a series of homework assignments. While writing one of my manuscripts, I picked up a habit that helps me keep the plot moving and avoid extraneous exposition.
When I start writing a new project on my laptop, I get out a good old-fashioned, empty notebook. I write the working title of my new project on the cover. Sometimes that working title is more like 'the one about the frog thing' than The Princess Frog, but it's just an identifier so there's no pressure to live up to the title.
One of my pitfalls when it comes to exposition is characters. I generally write in first-person, but the protagonist tends to get a little too narrator-y when I introduce new characters. A simple writing rule for me is when I bring in a character, I grab the notebook before I write any more on the actual project and I put the character's name at the top of a new page. Then, I write all those things my protagonist wants to tell readers about that character. This way I won't have to cut it later and the character becomes developed enough that I can work with them as a real person. It also provides a handy reference so I don't have to go zooming back to page one to remember if that character was blond or had a bad temper.
I also use the notebook for potential scenes. With a small enough notebook, you can take it with you when you need to take a writer's walk around the writer's block. This notebook is not about putting absolutes down on the page. (That would be planning, and we don't do that.) It's about collecting ideas, settings, smells, and sounds that might make it into that project. You might find them helpful for a different project too. It's like a writer's journal but focused on one project instead of those seven other ones you've started.
The notebook becomes a reference tool and a wrecking ball for writer's block. If you're writing yourself into a corner, take out the notebook and page through for the next idea to spark a scene. If it was inspiring enough to write it down once, it will inspire you again.
You're not really planning since that would imply work done before you start writing; you're just collecting stuff and writing down information you need and you want to appear on the page. You want these things to come out through storytelling. Let the characters expose themselves without your narrator inserting an exposé. This method helped me cut down on the fat that would have required trimming later.
Do you have other methods for not planning but keeping the story tight? Share them in the comments.