Well, welcome to GTP, but this is posted in the wrong forum...it shoulda been in the GT3 forum, but maybe a mod can move it for you.
As for your question, the best way(I've found) is to press the D-pad in the direction opposite the car you want, if you know where it's going to stop. I've also heard that pressing X on the car as it's lighted works, but it didn't work for me.
Hope that helps.