mmm....are you using th Formula GT car? because that's the one I used and I pitted less often than the AI.
I used TCS to "4", but It would be useless if you don't drive properly, like you said before.
Everytime you corner, brake before turning, or in other words, brake in a straight line, AND brake early.
There's also a term that will help you not only on tyre wear but on efficient driving...try to remember
SLOW IN, FAST OUT.
The FGT has very good brakes but If you are a late braker, then your tyres will become red quicker.
So, don't brake while you hit the apex, and don't push the gas too early, only when you have gotten out of the corner.
Use trottle control efficiently on every corner and do it with smoothness and ease. Try to imagine classical music on your head so you get a feeling of perfection and not reckless driving.
I didn't give major tunnig changes to the car, but I do recommend turning TCS up a few clicks.
If you're still having problems with pitting, try to follow the AI so you can get a better racing line. Not that they're the best at it, but if they are beating you, then you might want to follow their driving habits as well.
Try to search in the GT4 Tunning forum for "tyre wear" for further info on how to get the most out of your tyres.
Ohh and one more thing...if you are not using the FGT and your driving a LMP car instead, try to get the R5 tyres for it, and definetely try to get the FGT by winning the 24 Hours Nurburgring endurance race.
I hope this helped out a little bit
Ciao!