The go-to choice for mounting about anything would be
http://www.rammount.com in my opinion. You need to mount one "permanent" part to the bike, a small black rubber ball. You then clamp the arm etc onto that ball when you need the mount on the bike.
The mounting ball can be attached just about anywhere you can fit one. They have an option to mount it to the mirror stem, to the brake cylinder cover, replace half of the clutch grip clamp to the bars with their version that has one or two mounting points, the good old fashioned steering bar clamp etc. Personally I replaced two of the bolts that hold my handlebars in place with their M8 bolts that have the ball on them so I have two pretty discreet mounting points on the bike.
As for the part that actually holds the phone, their X-Grip lineup is good, I use one of those.
http://www.rammount.com/products/xgrip Though I always mount it horizontally so the phone rests on two of the gripping arms instead of just being held by the pressure as it would vertically.
Used that setup now for two years too, and it has worked pretty much flawlessly. I got a Lifeproof.com case for my iPhone, so it's now IP68 water/dustproof while in the case. Did an 8 hour run in a Norwegian downpour and the phone remained bone dry (doing GPS duties with the Tomtom GPS app).