How do you plan a trip?

Very carefully. Everyone is different, personal preferences but most have alot of similarities. I set my destination(s) then plan the route. Based on how many miles I want to drive per day I research the route for various attractions (including low bridges/tunnels, steep grades, and elevation) and where to stay for a night or however many nights. I draw my route on actual paper maps. I plan the rest stops, fuel stops, and food stops. Anyplace I enter must have suitable exit route, especially gas stations. I plot every location using GPS coordinates then upload to my Garmin. I use two GPS units and in some cases maps for those areas where GPS loses sync with satellites.

I then research road construction and plan backup routes if necessary.
Make reservations, confirm, confirm again when enroute. When I pull in the red carpet had better be rolled out (just kidding). Sometimes will have backup camp site somewhere near just in case. I can't fit in all campgrounds and I don't dry camp either.

If it takes me 3 or 4 days of driving to get to my primary destination then as a rule I prefer to stay at least 5 nights.

I once traveled 581 miles in one day with the kids and fiver and swore I'd never ever do that again. I slept 12 hrs the next day.

I'll have 10 weeks off next summer so I've already started planning my trip.

Chance favors the prepared mind.