One of the best tools I've ever found for planning cross country flights is one that it seems few people know about.
Go to weather.aero, where you'll find an experimental adds site from faa / ncar (national center for atmospheric research).
The site is good, but check out the flight path tool under desktop apps. It is a java app that works on mac or pc.
The brilliance is that it gives you access to the 3 dimensional current/forecast model of the atmosphere. You put in a route of flight, and you can see VERTICAL cross sections as well as an area view for the weather aspect you choose. If you turn on relative humidity, for instance, you can see where the cloud layers and tops are forecast. It is brilliant.
Anyone else been using this? it is the only way I've found to plan long trips particularly where there is imc with chances of icing I'd like to avoid...
Greg