Watch the map for reported locations of single vessels or convoys. Then go there using this technique:
http://www.subsim.com/radioroom/showthread.php?t=88961
The intercept drawing in synopsis:
Or else be in the right spot (traffic chokepoint) and make sure your crew looks around. Or ever so often dive to periscope depth and do a hydrophone listen yourself. That allows much further detection. (atleast 2x)