It's easy to read all the various tourist guides and 'oh, I want to go there and there and there', more difficult (on a big trip) to remember all those points that we hoped to visit. But I have a system.
I do my planning with a variety of research methods: guide books, websites, free PDF booklets downloaded from official tourism board websites. But I need to put all the sights in order so that we don't drive past them without remembering to stop, and so that we do them in a logical order.
At a European gas station we bought a great map book.
With this atlas I plan. I cut 'post-it' note stickers into little arrows and I mark points on the map. Different colours have different meanings (attraction importance, scenic route, campsite), and I make notes on a note sized 'post-it' and attach it to the page. The notes remind me what's at the arrow and where I have more info. Here's a sample.