I like travelling without plans but I don't want to be caught without a place to sleep
I usually book a couple of nights at the beginning of a trip and then book as I go. You may end up in one area that you really like and want to stay a few days, conversely you may decide that what you thought would be a great place to stay, really isnt.
Of course, if you are travelling over a major holiday time then you are going to have to book in advance.
The B&B concept in the US is not the same as in Europe. A B&B can be priced more than a budget motel and they usually charge per person and not per room. This makes them expensive in comparison to some of the better cheaper chains that criss-cross the States such as Econolodge, Comfort Inns, Best Western, Days Inn, Red Roof Inns, and my favourite, Motel Six.
Another alternative would be to camp. http://koa.com/ is a great website which shows you the locations of camps - most of which seem to have an option to hire a lodge/kabin at all different price levels.
You could also look at hostels such as http://www.hiusa.org/ or if you wanted free, then www.couchsurfing.org You could also check out listings on www.craigslist.com almost 10 years ago
Motels are a great option in the states if you want to travel without too much planning.
If you are travelling you can pick up motel guides at roadside stops along most highways.
Roomsaver ( http://www.roomsaver.com/ ) is a pretty good one.
Travelling in groups of 4 can save you a lot of money too. Lots of motels have rooms for 4 people with 2 double beds for around $60 which is much cheaper than a lot of other accommodation.
If you have plans then couchsurfing like Sam can be great, but you really need to arrange that sort of stuff beforehand. almost 10 years ago