They make really nice and very cheap arm band iPhone holders that have velcro, and wrap around you arm, and they hold the iPhone VERY tightly so it can't fall out. Get them really cheaply at Walmart or Target for less than $10 bucks.
As a runner, I strap mine in upside down for that very reason. I love running in a light rain and didn't want mine to get messed up.
I've been doing this withthe same phone for over 2 years now, and have no issues at all.
Happy running! Run to Live, Live to Run!