Fluff the feathers. If your feather bed topper has become lumpy, it's because the filling has become packed down by your body weight. Letting a feather bed topper tumble about in a large dryer will fluff the feathers. If your dryer is not large enough, take your down topper to the nearest laundromat; either way, 30 minutes on a cool setting will restore your lumpy topper to its formerly fluffy state.
Flip and turn the mattress. Just as standard mattress manufacturers recommend you flip and turn your mattress on a regular basis, you should flip and turn your feather mattress when it starts to feel lumpy. Flipping your feather bed mattress so you are sleeping on the other side will redistribute the feathers and eliminate lumpiness; turning the mattress so your feet will now rest where your pillow used to be accomplishes the same goal.
Let nature do the job. Choose a sunny, windy day and lay your feather mattress topper across at least two lines on your outdoor clothesline. The sun ensures no rain will ruin your down topper and the wind will freshen your topper and fluff the feathers at the same time.
Manually redistribute the feathers. While the mattress or mattress topper is still on your bed, remove the sheets and mattress pad. Use your hands and forearms to smooth out the lumps and redistribute the feathers evenly. Repeat as needed.