You'll have to literally save this one for a rainy day. This little trick requires one of my favorite iPhone apps, Slow Shutter Cam (iTunes link), and an evening rain shower. Make sure you set the camera to capture in "Light Trail" mode, set the Sensitivity (shutter speed) to 1 or 1/2 a second, and turn your LED flash on. Oh and be smart... bring something to dry yourself and your phone off. Both you and it are going to get soaked.