Ok lets start with the "Smooth Move". What I suggest is that when ever you are in a normal oval / tri-oval track that your thumb never ever comes off of the left anologe stick. That way you can keep it smoother and experiment to find what position the stick should be to take the corner best. It also stops the car from "Snapping" into position which can ultimatly: End you in a spin, or have you ploughing into an ally or rival.
Next we have the setups. Although there is not a very detailed guide to setups, (although I highly recomend Xp132476's NASCAR 06 FAQ. It has several tracks that are "unique") you should experiment on different setups. This is what I would do in a normal race. First I practise, then quit, look at board to see practise times, if I'm happy qualify, if not tinker about with the car and ask yourself several questions. Did the car want to get loose at the end of the corner? Yes. The add a wedge or increase downforce. Etc. Etc.
Lastly we have Allies. Try to make friends, cause the friendlier they are the more drft you'll get and the faster you can go. If you hit them by accident and they turn to rival, sit behind them and push. (DO NOT ACTIVATE INTIMADATOR(R) OR SHARE DRAFT). You will start to bumb draft them which is +15 in points, and after a while will have him an ally again.
I hope you can learn from this guide and become the next NEXTEL CUP CHAMPION