While you can use packing cubes for organization inside a duffel bag, they do take up a lot of space because of the way they’re shaped, i.e. rectangular blocks. If you must use them, choose long and narrow ones instead of bulky broad ones. If not using cubes, packing a duffel is all about maximizing the hard to reach bottom corners. Pack things like socks and underwear in those corners first. If you don’t have a separate shoe pocket in the bag, separate the pair, pack each one in its own shoe bag, and place each bag vertically right against the sides of the duffel. The remaining center space in the main compartment is now left for all your other belongings. All your larger items of clothing should be rolled up instead of folded. Not only does this take up less space, but it minimizes wrinkles, which can be a problem when packing a duffel instead of a suitcase. Pack your rolls of garments lengthwise inside the main compartment instead of crosswise. The latter way wastes more space and you don’t want the bag to bulge out. Lift the bag up a little and shake it so that everything packed so far settles and you can make a little more space on top for the last items, like a toiletry bag. If you’re taking the duffel as a carry on, then this needs to be a TSA-approved toiletry bag.