Here's what i do:
First thing i don't do is sit ahead of the convoy, near the track of the lead escort. If you fall into his active sonars beam, he will find you most of the time.
What ill do is sit ahead off the convoys track, but slighly off of it, to keep clear of the lead escort. My intent is to move in closer to the convoy, once the lead escort has crossed my bow, before the side guarding escort happens up on my current position:
Like so...
By staying out of the lead escorts path by a 2,000-3,000 meters or so, ive avoided him detecting me, and ive slipped in behind him. Timing here is crucial, as you also have the side escort to worry about. You need to move, but not so fast that you make alot of noise. Do this right, and you'll have the lead escort way ahead of the convoy somewhere, the side escort somewhere behind you, but effectly a non factor until your torpedos start hitting, and a nice field of fire with overlapping targets.
Naturally this varies. If the side escort has pulled in closer to the convoy , then you have a problem. Sometimes ill improvise and point my bow at the lead escort to minimize my profile,(cause ill be in his sonar beam) and then swing a hard port or starboard the instant he passes me so i can bring my tubes to bear on the convoys track. Which also puts me in a position similar to the picture above, only its alot more risky since the escorts are much closer in that scenario.