[Beknatok]

To add to the list:

Rain gear
A portable tire compressor (Slime makes a motorcycle sized one)
Extra fuses (go read ant_mb's latest post about how he just blew a few)
A small flashlight
Painkillers (sucks getting a headache on the road)

Hydration systems are your friend. Don't wait till you're thirsty to drink... take sips every now and then, to maintain your body's hydration levels.

I also cannot emphasize the advice of eating well, or more specifically, eating right. At least for me, I cannot do heavy, greasy meals while on road trips, as they'll make me more fatigued. I eat carefully and watch my portions. I also keep some trail-mix with me, in case I need a healthier sugar fix.