I started out knowing that I wanted to be up north all summer and head south all winter. I have a notebook (I write everything down) and researched top must sees or must dos of each state. (We're skipping the NE states that we've already been to) I did lots of Google searches. I made a tentative plan and then added a lot to it after getting advice here on this forum. If you read through this thread and the one on bucket lists, people have awesome suggestions. That's where I started.
There's so much to cram into such a short period of time lol, you realize how much there is to see and do once you try to plan it all in a 10 month period. You also need to remember to plan on time for laundry and "house keeping" like vehicle maintenance. I have a cram packed schedule, HOWEVER, I am using that as a suggestion and over planned instead of under planned. So there are things on our itinerary that we will probably not wind up doing if I need a down day or aren't up to driving, if someone gets sick (there are 7 of us so it's bound to happen at some point), if we have vehicle issues, if we decide to stay somewhere longer than planned to see more there, if we hit bad weather, if places are closed etc etc etc.
Also, like Tom said (and he is an AWESOME source of information, he's been a huge help to me over and over again) I also don't re-cross my path.