There are a of couple options! You can use your hands to push the down around or a tennis racket to smack the down.
Pro tip! You can also toss your bag in the dryer on no-heat (air fluff) and add four clean tennis balls, the static created by the tennis balls will help separate the clumps of down. Note, this may take a few cycles. Don't panic if nothing has happened in a 2o minute cycle, just take the bag out, hand-fluff, and try again in the dryer with no heat.
Do NOT put your sleeping bag in the wash.