You have to learn to take advantage of "radio contact reports". The realy lousy thing about the game is that these reports don't realy come in through the radio operator (as they should), hence he won't report anything to you... rather, you have to keep your eyes glued to the nav-map at all times and whenever an enemy ship misteriously pops up way even though it is well beyond visual range (and this will happen even with the map auto update turned off), then that means that you just got a "hot tip" on an enemy ship through the radio.
That's where most of my tonnage comes from
For more info read this link:
http://subsim.com/phpBB/viewtopic.php?t=47980