Making reservations ahead of time may crimp your Kerouac style, but it will also save you money. Instead of pulling in to a hotel desperate, look online for deals before you go at sites like Roomsaver.com and Travelcoupons.com. This is doubly important if you're road-tripping on a holiday weekend, when making reservations beforehand may not only ensure the best price, but can also protect you from having to spend the night in the backseat of your car.
Better yet, camp. If you're willing to rough it, consider pulling off the road in a national forest or Bureau of Land Management area and unfurl your bedroll wherever you see fit (after checking on restrictions and permit requirements with the local Ranger District or Park Service Office). The Web site Boondocking.org lists GPS coordinates of free campgrounds around the United States.
Or even better, visit a Web site like Couchsurfing.org, Airbnb.com or Tripping.com, where weary travelers connect with willing hosts. In addition to sleeping for free, you'll do your part to make the world a slightly friendlier place.
And by the way, cheap showers can also be found at truck stops.