I have done several different things....they all work.
On the little Ninja, I used a Cigarette Lighter socket that I got at the Auto Parts store. It is a flat mounting surface, and the foam mounting tape holds it tight to the backside of the fairing. You just plug in your cigarette lighter charger.....most likely they are not very waterproof...but neither is my phone.
On the Honda ST1100 I have plenty of room, and I used one of these. They are a bit bulky and have thick wiring, and they come in both single or dual charger models.
http://www.ebay.com/itm/Motorcycle-Mobile-Phone-GPS-Waterproof-Dual-USB-Power...On the Cafe bike I just couldn't find a good place to mount the phone or the charger. So I bought a battery back up and when the phone in my pocket runs down......I just plug it in to this back up battery. I can get about 4 charges and it keeps me in phone all day long. I have a Bluetooth headset, so I can answer calls or hear the GPS navigation with the phone in my pocket. The only downside is I have to stop and get the phone out of my pocket to dial calls or to change the GPS navigation. The really cool part about this is the thing was cheap, requires nothing to be done on the bike, and it also works when camping or hiking.
http://www.walmart.com/ip/5600mAh-Universal-Power-Bank-Backup-External-Batter...With any of the above....you might consider a good tool roll mounted on the top of the forks, or a small tank bag.