PB - Famine's advise on the licence test is a good tip, and well worth doing.
Personally I got to grips with the 'ring in time trials rather than races, as I found it allows you to concentrate on the line without the distraction of other cars or the pressure trying to catch another car.
Start with a car you know well and find predictable in handling (after all you don't need the car working against you as well), and to start with of a low to medium power to weight ratio (but nothing to light it can be a pain to start with over the 'rings many bumps).
Drive the track at a moderate pace and just keep going, pushing a little more each and every lap, as unfortunately practice is the key to the 'ring. As I'm sure you have found some parts of the track will not allow you to push too hard or they will bite back, combined with many of the corners being blind and experience and practice are you best allies here.
However the following can also help...
http://www.bmw.com/generic/com/en/fascination/bmwm/automobiles/specials/_pdf/bmwm_nordschleife.pdf
...the above link will download a 25 page guide to the 'ring, covering every corner in detail. It was put together by BMW, Sport Auto and BMW works driver Jorg Muller, each and every line is explained as are braking and clipping points. You may find it helps get the line right on some of the trickier complexes.
I also found that watching as many real in-car videos of the track as I could helps pick up the right lines through some sections, with this always a favourite...
...its a six minute 40 second (approx) lap of the 'ring by Derek Bell in a group C Porsche 956. I'm fortunate enough to have this on DVD, but the you tube version is not bad.
Or this one, which is a Radical SR8
Keep practising, its worth it.
Regards
Scaff