I always put my backpack or briefcase under the seat in front of me to spare overhead room for the people who need it. I did the trick that was mentioned earlier by pulling it out later and putting my legs under the seat in front of me.

I heard a new announcement on a DL flight last week where they said that all aisles must be clear at all times. They then said that this includes the space between seats; that this is also an aisle and your bag must remain under your seat throughout the flight or you are blocking an aisle. Fortunately, they never noticed.