I do this debate all the time, as I ride alone. If I come off my horse, I want my phone, but I don't want to fall on it and break it. I tend to leave it at the gate of the arena, as I think in most circumstances I could probably drag myself to it.
If I ride in jeans- it goes in my pockets. Otherwise, I have a case for my iphone5 that works as like an exercise band for joggers. I put my phone in the sleeve and it attaches via velcro onto my arm. Its pretty nifty. Like I said, intended for joggers, but works just as well for me.
I have a friend who has a case that attaches to the belt loops of her breaches. That is another option I suppose!
Though I don't carry a phone while riding, 'cause about 99% of the places I ride don't have cell service*, I carry wallet & camera in a pouch on an elastic belt. Designed for runners &c - I got mine at REI.
*Though oddly enough, one of the people I was riding with on a camping trip was able to get service from the Summit Trail in the South Warner Wilderness, which is a long, long way from anywhere.
Why not use baling twine to make a pocket net or buy use one of those bags sunglasses come in for it then hook it to one of those necklace key straps and wear it around your neck? If nothing else you can add velcro on back of phone(bag) and shoulder of shirt to keep it from bouncing around while being connected to the neck strap..
I stick mine in a pocket when I wear jeans sometimes and the other times I stick it in my boot. Im used to things in my boot though since I always keep a knife there when I ride. When I am wearing half chaps I can stick in in the outside of my leg and it stays pretty well. I have an older iPhone. Posted via Mobile Device
When you say you put it in your bra are you talking between the boobs? I put mine in the bra on the side (facing out...you know, sweat) and never notice it even though my phone is enormous (Galaxy Note II). Sometimes I forget it's there until it vibrates, haha. But my bosoms are, uh, pretty ample, so it might be uncomfortable for a normal person.