How can I keep the feathers from poking through the cover of my featherbed?

Make sure that the duvet covering is of superior quality. A down-proof 100% cotton ticking with 230 threads per inch is best. Some cheaper ticking has a low thread count disguised by smoothly pressed sizing or filler which soon disappears with use. The down will escape through this porous cotton. Some very high-thread-count ticking is too fine and wears through quickly.

http://www.oldeuropeduvet.com/choosing_your_down_duvet.htm