Ok...I consider myself a Ring pro now...I've put over 500 Miles on at least 3 different cars, and driven approximately 200 different cars for at least 3-5 laps around the Ring...
Everything from a Honda CRX on N1 tires (10'00 flat) to a Polyphony Digital (5'20, with THREE complete mid air spins...nuts, that) and all sorts of cars in between.
I find my personal favorite choice completely invalidates what someone said earlier, "the ring is not kind to cars that like to oversteer"
My 550 Mile Ruf RGT on N2/N1's disagrees thoroughly, and I can now drift every single turn on the course like a bad ass.
Still got a few more laps before I can get a totally clean lap with a BTR on N2/N1's....but hey, that car is like Ice Skating.
The Ring in reverse...man...you really don't appreciate just how long the downhill stretches are, til you drive it in reverse and see that most of the track is a gentle downhill slope, which is why a lot of the turns are tricky...it's much easier to lose traction and slide when you're taking a 65 degree corner at 120 Mph downhill than it is uphill or on flat ground.
I won't try to give you specific corner tips, as I have to rely on reflexes to take the track gripping anymore...now I'm just obsessed with trying to link as many turns as I can.
Here is the tip that I give to the people in the Nurburgring Diary thread on GT4's Gamefaq's board: Pick a car you like and want to improve your lap time with. Change the Oil, Turn off the Aids, Put N1 tires on it.
Drive until you can run a clean lap with N1's (it will be slow...slower than the car runs in real life, as you have to brake earlier, take corners gently, and be careful when you accelerate) and by clean, I mean not even letting more than 2 tires go off, and no wall kisses.
After that, move up to N2's or N3's...lap a few times...slap on the S2's or S3's the car came with...you will find yourself undercooking corners at first, then you will see just how quickly you can really take things, and I guarantee you will drop 10 or more seconds from your first lap after hopping on S2's...you feel like you're glued down.
Drive lots of different cars, don't be afraid to oversteer a little bit...
and if you really want a wild challenge...pick up a Ruf RGT, and but N2/N1 tires on it and try to run a clean lap....it is WILD.
Edit: I found a good yardstick for your skill against real world drivers.
Get a BMW M3 GTR Racecar, put R1 tires on it (as the tires the cars come with tend to be about two grades stickier than real life tires) and try to run a 7'00 Flat.
Hans Stuck ran a 7'21 during that Qualifying run...if he was going flat out, I wouldn't be surprised to see him push a 6'40...but a 7'00 flat is a DAMN good lap for that car on Racing Superhard tires...no aids, no mods...just you, your road carving tool, and 13 miles of death.