Getting there on foot is pretty inexpensive and allows you to relax a bit more than other modes of transpo. Even though I love driving, I really prefer to drive something fun, rather than boring.
I left from Zürich but I'll start this tutorial in Stuttgart.
The train drops you off at Stuttgart Hbf. Once there, head outside to the bus stop and buy a ticket in the machine. The ticket you want is #245, it should cost you about 2.35 Euro. Its a one way ticket, but the station at Porsche also has a machine for you to buy one. They are only good one direction and only for two hours. There is also a daily pass you can buy, but the break even point is about 3 one-way trips. If you were doing the MB tour as well, it'd be worth it.
Once you have your ticket in hand head to Gleis(track) 3. There you will find a large set of stairs that go underground to the subway. Head down there. Once down there, you should be looking at Gleis 101 and 102. Look for one of these blue signs:
You are looking to head generally north from the hbf so you want track 102. Wait for the S6 train to Weil der Stadt. The sign above has all the info you need. Once on the train you are heading to Nuewirtshaus (Porscheplatz). It is the stop after Zuffenhausen. Here is a picture of the stop you want:
(that red and silver thing in the bottom right of the pic above is the ticket machine, they all look alike) The train drops you off in the shadow of the new museum which I'd guess should be complete by spring 2009. You are also across a small parking lot from the Porsche Zentrum (Center). Once the Museum is complete, the rest of this post is worthless, but it wasn't totally smooth for me to get to the small museum so here are some directions to that.
From the Zentrum put yourself between the Zentrum and the New museum. Museum on your right, Zentrum to your left. You are now looking in the direction you want to walk. Go across the street heading in a straight line. Once across the street you should see a sign that has an arrow pointing down the street I just lined you up on. From the Porscheplatz (the roundabout you just crossed) walk 300 meters. if you are not sure how far that is, just walk to the next street light. You'll see this sign.
Turn left and walk about 80 meters. On the right side of the street you will see this sign.
Turn left. you can now either enter the museum to your front right about 60meters or go straight until the guards stop you. Go inside, and enjoy.
It's small, tucked away, and totally easy to miss in a car. People in this part of town don't speak much English.
I actually asked a guy, "Wo bist du, Porsche museum." Ya, I know. "Where do you live, Porsche Museum?" He smiled but got the point.
Enjoy your travels.