Well, for one, where are you patrolling? On the way to and from your assigned grid, you might want to do a bit of fishing along the major shipping routes. (Check the pullout map on the top left of the navigation map station) Periodic sound checks can only help your chances further. And as Maillemaker said, AM52 and the Rockall banks are good places to check, if you get assigned near there, and I've caught convoys all around BF17 and the northeast corner of the BE sector.
Also, I'd say that up until 1941, you can expect to see quite a few neutrals. This past convoy (December 1941) I accidentally fired upon a French merchant that was travelling with a British large cargo. (Thankfully it didn't sink, even though I probably sent several dozen rounds into it)
Finding something to sink is probably half the game, and the more you check around the better your chances are of finding a fat tanker or somesuch.
