Check out this link. Will answer a lot of questions.
http://www.subsim.com/radioroom/showthread.php?t=146795.
Best advice is to run silent and run deep and run slow.
Change your direction and depth often. When a DD makes a run you can go to flank speed for a few seconds and than cut power and coast changing course as well.
The very best advice is to not get detected in the first place.
Magic