My best aquascape plans have come from hours of sitting in front of a tank and letting my mind wander. Often I'll daydream about a particular aquascape that I want to do and let it form and mature in my head before I ever do it. I wouldn't call it purposeful planning though, it's definately a form of pleasant distraction from real life.
In the past, when doing an installation for a customer I didn't have the luxury of weeks. I would do a lot of sketching then if I could. Most of the time though I would have to work with whatever I had on hand at the moment and pretty much just improvise. In a lot of ways I like this method the best and the majority of my work, personal or professional, is done this way. Especially when trying for a natural vs. an aquatic garden look.