A really useful site with which to get a big picture look at the upper air situation is Cameron Beccario's Earthwinds. It is incredibly dynamic; you can rotate the globe around with your mouse and look at the situation from any point on the globe. In the lower left corner, the menu is suppressed under the word "Earth"...click on it, and the menu opens. If you click on any point on the map, a display will open in the lower left providing a digital readout of whatever parameters you have selected at that particular point; the units can be changed to taste.

I usually look at the winds at 250 mb, which gives me a good picture of the day at cruise altitude, and then look at the total water content (TWC) at the 700 mb level. This gives me a sense of how wet the atmosphere is around 10,000 feet as well as the flow at that level, which can be influential for thunderstorms. Next, I'll look at the 850 mb level and turn on the CAPE option, which will produce a color coded map of the Convective Available Potential Energy at any point that I click on the map. This information prepares me for the next several sites.