Another solution is to wire in a power adapter connected to your motocycle's battery that can charge them via a 12v>5v USB adapter. It would be by far the simplest approach and probably a cheaper one than buying batteries and such. Make sure to add a fuse and a cutoff switch.
Unless you're in an area that gets lots of sun, a solar panel probably won't be able to pay for itself with short term use as they are relatively expensive. You can also look at projects like the Mintybuck:
The Mintybuck is a DC-DC Switching converter (Higher Efficiency than a Linear Regulator) that runs off Li-Ion batteries based off of Ladyada's Mintyboost project. You can pick up Li-Ion batteries from plenty of vendors and even laptop batteries. 18650's are between $3-8 each and pack plenty of power if connected together.
If your mind is set and you want a solar charger I suggest just buying one pre-made from a company such as Brunton. You will need to have a smart charger for the battery as well as a proper DC-DC converter as the voltage can drop in the solar panels which means you'll need a SEPIC configuration converter.
A great generic DC-DC converter IC you can use to get your hands dirty would probably be the 34063, or the LT1302 if you only want 5v output. Lots of information on these particular chips.
Something to keep in mind is that USB powered devices can draw more current than the standard 500ma upwards of 1A. These devices usually sense the device they are attached to by shorting the data leads or applying a voltage/resistor to them. You can find more out about that here: http://www.ladyada.net/make/mintyboost/.