Your nose will run if it's irritated, and some noses are more susceptible to some irritants than others. Environmental irritants such as pollution, dust and tobacco smoke -- even chemicals and perfumes -- are common causes of a runny nose. Unfortunately, you can't always avoid these things, especially if you live in a smoggy city or with a smoker. Still, there are things you can do to help.
Keep your nasal passages moist with saline spray or by using a Neti pot. Running a humidifier can help, too, although you need to make sure it's clean so it's not pumping irritants into the air along with its soothing mist. And drinking plenty of fluids will ensure your sinuses stay moist, among other benefits.
If you know something irritates your nose, avoid it altogether if possible.