If you run with snorkel up and desiel engines running.. Also have both periscopes up.. one at 0, the other at 180. In daylight "aircraft spotted!" - Press D or C to dive and Ctrl Page Down (and respectively for Observation persicope) to auto-lower your scope(s). I have always managed to duck under before the bombs hit.
The snorkel makes a nice signiature for the Allied RADAR.
The best option is to look at the map provided with the game and go to the Atlantic between Britian and North America. You can find some nice spots that are far from any air coverage and along large convoy routes. They also often have a depth of over 1000 meters. If you have to, dive near crush depth where they can't find you. Also keep in mind that cold water (and air) is denser, thus harder for anything to penetrate (hydrophone, SONAR, etc). Pick a nice ice-cold spot generally around Greenland and Iceland. You will have plenty of large convoys to chew up.
When need-be, run on the surface at night to the artic and grab your external torpedoes... just to return to your sweet spots.
I would not use your RADAR unless in a blinding storm. They have RADAR detectors too!
If you are running GWX3.0 , there are resupply ships and Milchecows that visit said area time to time. You could just stay out there. When you dock at a resupply ship.. it takes you back to the Officer's Office, but everything is disabled. You can only just go out on another patrol (refitted, resupplied, refueled). I have done this, as opposed to spending alot of time and fuel traveling to and from L'Orient. (A hazardous port). Also in GWX is Wolfpacks... you can sometimes find a pair of U-boats to join, and form an "Atlantic Wall".
Good Luck!
