I would just scrape off those sagged areas with a sharp paint scraper and repaint.
Thinning your paint is good practice in my books because it BOTH lowers the viscosity of the paint so that it self levels faster and dilutes the paint so that it takes more time for the thinners to evaporate, thereby creating better conditions for the paint to self level on horizontal surfaces so that you don't see any brush strokes.
The problem is that by thinning latex or oil based paints, you also create better conditions for the paint to sag as it dries on vertical surfaces.
I've been told that using a paint conditioner like the Flood Company's "Penetrol" for oil based paints and "Floetrol" for latex paints thins the paint without lowering it's viscosity, so that you prevent sagging in the paint on vertical surfaces. However, I've never tried either product when painting vertical surfaces to see if it works that way. If you try it, and it doesn't work, then keep thinning your paint, but apply thinner coats to eliminate sagging.