When attacking a convoy: Let the first line of ships go by unscathed. This will create a shield of noise between the U-Boat and the lead escort. Chances are the lead escort won't even be able to hear your launching of torpedoes. Later, your high speed dash and wiggle-waggle while go you go deep will also be more difficult to detect.
In most cases I've used this tactic, escorts never bothered me.
