Charging from a PC USB port will be damn slow due to the poor amperage (500 mA). Using the provided charger (1,000 mA) will definitely charge faster.
Unplugging and re-plugging right after a full charge doesn't make it charge more, it merely makes the device verify if the battery can still be charged and that takes some (useless) time.
The battery will only seize its full capacity after you charge and discharge it fully for about 7 times. Keep in mind that keeping Data/GPS/WiFi/etc on all the time will increase the battery drain. Also update Google Maps to the latest version (version 5.0, which comes preinstalled in OB, is known to enter a loop sometimes and eat up all the battery in a matter of hours).
If you need to charge from your car, I strongly suggest Griffin's PowerJolt (2,100 mA), that's enough even for a tablet: http://store.griffintechnology.com/powerjolt-ipad
(it will charge ANY USB device - and FAST).