Once the run has dried, scrape it off with a razor blade or putty knife. Try to keep the tool level to the surface of the wall and use light pressure. Be careful not to scrape so hard that you scratch into the wall.
After you scrape the paint runoff, lightly sand the area in a circular motion. Sanding is complete when the edges you created with the razor blade or putty scraper are smooth.
Take a damp cloth and wipe away the dust created by the sandpaper. Let the area dry.
Repaint your repaired area using very thin layers of paint. Let each coat dry thoroughly before applying the next. Continue to apply thin coats of paint until the surface is level with the surrounding area.
Inspect the rest of the paint job for additional runs that can be fixed using the above steps.
Step back periodically as you paint to check the area as a whole. This helps you see potential drips before they dry.