Step 1: Get the Goods
1. A 9v or greater battery
2. A 9v battery clip
3. A lm7805 or lp2954 5v regulator (Sampled from Maxim-Dallas)
4. A female Usb port (also sampled but from keyeleco.com)

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input voltage = 9, output voltage = 5, say your drawing 500ma, a full load for any USB port, then your looking at (9-5)x0.5 = 2 watts of heat. the LM7805 by itself will cook itself to ~120C before internals shutdown
without a heatsink you will start to smell the solder smoke pretty fast.
Last time I tried something like this I thermally epoxied the LM7805 to a 3 watt capable heatsink, tied the input pin to a 12V source, the output pin to a 5V 40mm fan mounted to the heatsink and made myself a little space heater _
if the heat gets to you, an alternative option would be a zener diode of some sort...something like 1N4732A with ~193mA sustained current. Zeners diodes are cheap you could place several in parallel to share the overflow current once whatever your charging are up to ~5volts.... I'm sure theres an instructable for that here somewhere
iPod charger using transistor
from http://electronician.blogspot.com/2010/11/simple-ipod-charger-using-6v-to-9v.html
go to this website for the one that works with new ipod models and iphones
www.tzywen.com/modules.php?name=News&file=article&sid=683
enjoy