Step 1: Learn your car's specs
Go to http://www.teslamotors.com/models and play with the settings to understand how various factors impact range.
Remember that these are only estimates: other factors (e.g. weight, number of occupants, elevation gain, the tumbleweed that got stuck in the grille two states ago) will impact range as well. You always want to give yourself a margin of safety.
Step 2: Map your route along the Supercharger path
Go to http://www.teslamotors.com/supercharger, pull up your favorite Maps app of choice, and establish your route along the path of Superchargers.
Step 3: Figure out the distance between each Supercharger
Just use Google maps for this. The point of this is to ensure that every leg of your journey is actually within the range of your car, and to give you an idea of how much you need to charge up.
If you only have 80 miles to the next Supercharger, charge up for 20 minutes to about 80% and hit the road.
If you have 215 miles to the next Supercharger, you probably want to charge up to 100% and make sure you don't go much over 70MPH.
Step 4: Make your packing list
What to pack is obviously critical, and deserves its own full post. Check back for that soon.