I usually motor on at ~190 and flank at an oblique angle towards the DE/DDs until they're within 800m. Once I see that they look like they're coming straight for me, I order a hard rudder towards my target, then cut engines. The drift until you hit 0 knots is usually enough to get you clear from where the depth charges are likely to hit; also, it suddenly lets your hydrophones update everything around me.
Once the DD starts turning away, I then open up at flank on the heading I want to go to and start motoring away, until the DD's within 300m. Wash, rinse and repeat until I'm within 1.5km of my target, then cut engines and drift to periscope depth. Fire all six torpedoes in two groups of three (one at where you think it's going to be, and one at one degree away from that bearing at each side), then dive to 30.
When firingat periscope depth, make sure you're at a complete stop before you raise the scope. If you're not, the wake will be a tell-tale sign. In calm seas, I usually raise scope at either 14 or 13 metres, to make sure that I put as little above the water as possible.
Stern tubes are usually very useful at this point too since enemy surface units are usually coming on at flank from behind, trying to catch you. I also find that using spectator view can be very helpful when pushing along at 30 metres because you can roughly tell whether the DE is going to nail you with the depth charges or not.
Now if only I could figure out how to KILL subs...
