To repair a run in your paint job, use a razor blade or putty knife and a piece of sand paper. First repair the run, then repaint the area.
Scrape the dried paint
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.
Sand the area
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.
Wipe off the dust
Take a damp cloth and wipe away the dust created by the sandpaper. Let the area dry.
Repaint the spot
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 paint job
Inspect the rest of the paint job for additional runs that can be fixed using the above steps.
Monitor the paint job as you go
Step back periodically as you paint to check the area as a whole. This helps you see potential drips before they dry.