Depends why it's runny. If it's something viral, take some cold/flu meds if needed, or just leave it be. If it's a histamine reaction, take a Benadryl.
Also, this is almost always a good suggestion when feeling unwell. Staying well hydrated, properly nourished, and getting rest are the best ways to naturally boost your own immune system.
Ugh...anyone know an easy way to get rid of runny nose? ╥﹏╥