1. If the paint is still fresh, scrape off excess with a knife or other tool.
2. Use a towel soaked in warm water to dab the stain.
3. Soak the denim in isopropyl rubbing alcohol (something you should definitely bring for first aid, cleaning electronics and, yes, for laundry).
4. Rub the fabric against itself, then scrub with a cleaning brush or bristle toothbrush, removing as much paint as possible.
5. Rinse the stain with warm water.
6. Toss the jeans in the washing machine and wash them separately in hot water.
In case of an emergency, you can also use dish soap, hand sanitizer, or hairspray. (The alcohol content of the latter two options helps break down the paint.)