There are a number of ways to fix paint runs. If you catch the pain run right away, you might be able to quickly wipe it off with a damp rag. If it is has dried, you can try using a single edge razor blade to shave it off. If you wait a short time period, you can sand off the paint runs and then touch up the area afterwards. Keeping a small paint brush nearby with a solvent will allow you to repair the paint runs as soon as you catch them.