[3Jane]

I keep my TC down to X128, it allows plenty of time to react to ship/aircraft sightings. I only moved up to X256 maximum when I bought a IXB to cover longer distances. You can sight a lot of potential targets on the way to your assigned area, so it's worth keepng the TC down. It's worth reaching your assigned grid area and completing the 24 Hrs there, any renown you can gather is worth it when it comes to upgrades in equipment or boat types.
Don't bother engaging aircraft, the damage and consiqential reduction to your hull integrity can severely limit your maximum diving depth when evading escorts.
Definitely use your hydrophone a lot.
Don't waste torpedos on small freighters, use the deck gun or if the weather prevents that, ignore them. Save the torpedos for larger targets. With unescorted large targets, I tend to use one torpedo and finsh it off with the deck gun. Only if the weather is bad do I use two. Make your fuel last, in a VIIB I found 9 knots to give the best range, 8 knots for a IXB. Be patient, you can see nothing for what seems like forever then have a nice convoy report by radio. This is certainly where being economical with torpedos pays off. Having only one or two shots left with a convoy nearby is certainly a frustration to avoid.