I rode from Colorado Springs, CO to Greenville, SC to Miami, FL in 3 days. In that time there was a 10% chance of rain and encountered a flash flood in Kansas, a fog bank in Colorado, a fuel line with safety attachment randomly disconnect, and a hurricane in Florida. This is my advice:
Sunscreen, Water, Gatorade - Even if your wearing jacket/pants, sunscreen the exposed parts. Water on the bike and a small Gatorade every fuel up if its above 90 degrees. Keep the caffeine and alcohol to a minimum if your going for distance and plan on spending all day on the road.
Wireless and wired headphones - if you have ever driven through Kansas and your only stops are for fuel every 2-3 hours, they will keep you sane.
Spare riding gear - If your wearing regular clothes and not expensive gear, bring an extra set to include shoes. Rain/flooding happens (even at 10%) and if that's later in the afternoon, it will make a very uncomfortable ride till your next destination.
Insurance/tow - This is one of the best things you can get as a just in case and its not expensive.
MSR bottle - Having some extra fuel on board will save you if your planning on taking the route less taken. I had my fuel line detach from the bottom of the tank in NC. I just plugged it back in and was good but it left me on E on the side of the road. The two MSR bottles saved me and got me to the next exit for fuel.
Snacks/food - If you feel like pulling over and admiring the view or you break down and are waiting on the tow truck and get hungry.
Credit card and cash - there isnt much in the world that cant be solved without money. It may not be cheap, but you can be rescued.
And on a personal note, I grabbed a 6 man tent and sleep pad from a friend and rented a few Campground of America spots along the way back. It was cheap, there were bathrooms, and they are generally close to civilization so I could grab anything I needed.
Adam