Maybe it´s not much, but it has worked great for me.
Step 1: Obtain a Cellphone Car Charger
First you need to find this thingy, once you´ve got it we can start building our charger. Any 12 V car charger with the correct entrance for your phone will work. You can buy them for cheap.
Step 2: Soldering
Find some electric wire or speaker wire that´s long enough to connect your charger with your battery. Solder one piece of wire using a soldering iron to any of the ground terminals as shown on the image. Then try to get the other piece of wire inside the 12 V terminal so that they touch and make a circuit for electricity to flow. Isolate your conecctions.
Step 3: Placement
Search in your motorcycle´s owner manual where the battery is, then decide where are you going to place your cellphone charger (I installed mine under my sit), you can use velcro or plastic fasteners.
Step 4: Connection to Battery
After you´ve placed the charger securly in your motorcycle, unfasten the screws on the terminals of your battery and insert the ground cable to the minus terminal and the other cable to the plus terminal. Fasten the screws again.
Step 5: Place the Other End of the Charger
Drive your other end of the cellphonecharger through your motorcycle, making shure that it doesn´t obstruct any moving part and that the connector stands where you want it, fasten it with plastic fasteners to the motorcycle´s internal structure. You can use a piece of plastic like the one shown in the image to hold the end on place.
Step 6: You Are Done
That´s it. There´s only one thing left. Try it !