You probably know already, but the tail really swings around a lot during power changes and can cause the pax in the rear end a great deal of discomfort if you are ham-fisted with the throttles. It's been a long time, but I seem to remember making slow power reductions in stages alternating with little shots of rudder trim (e.g. from cruise to idle power pull the throttle slowly back about 1/3 of the way while slowly adding rudder, stop, trim the pressure off, pull back the next 1/3 of the way, stop, trim a little more, etc until all the way to idle, or 30% torque, or whatever the target approach power setting may be). The same technique also helped immeasurably in the sim for maintaining heading and track while doing single-engine nonprecision approaches with stepdowns. Just start the power changes during intermediate leveloffs and descents early enough that you can move the throttles gently without blowing your altitudes and speeds.
Just mentally connect your throttle hand to the appropriate foot. They move at the same time at the same rate, with the autopilot on OR off, and of course much more pronouncedly while single-engine. Some guys would leave their feet on the floor with the autopilot on, pull the power back at cruise without touching the pedals, then use trim only to re-center the "trapezoid" as the airplane fishtailed, which you can't feel as much from the front. If you value your customers' comfort keep your feet on the pedals anytime the throttles move.
This of course applies to any large turboprop (or any prop plane period, really) including the DHC-8 100, 200, and 300. But it was my personal experience that the Q400 was much worse in that regard due to it's fuselage length and high power.