Thoughts like water
I use imagery a lot on the mat. It helps me stay focused when I can visualize what I'm trying to accomplish.
One that I use a lot when I'm on my mat is that I think of my thoughts as a glass of water. Our normal default mode as humans is to go 100 miles an hour, jump from one thing to the next all day and our water is sloshing everywhere making a mess. We don't even think about it. That's just how we're wired here. Imagine if you were literally trying to hold a full glass of water steady while grocery shopping or getting major stuff done at work or getting the kids ready for school.
I like to think of my thoughts during practice as the glass I'm trying to steady. When my thoughts run wild, my water is spilling, sloshing and full of bubbly mess. When I visualize the water stilling with each pose, my thoughts settle down too.
On the mat, we get a chance to practice. To play with steadying our glass. To learn how to do it better, more efficiently and have it last longer. To practice doing it in multiple kinds of conditions - in quiet, through noise, in movement, in stillness, on good days and bad days, etc.
We work toward stilling the bubbles because it's only then that we can see clearly inside and through the glass. It's impossible to look through our thoughts and see our clear self when the thoughts are running wild.