Gather the items you plan to take on the trip and lay them out next to the bag. Place cosmetics, toiletries and other small items in re-sealable plastic bags. This way you can gauge how much stuff you can feasibly take. You may have to cut down on some items; looking at everything at once helps you mentally organize what is and isn't essential.
Place the large and heavy items on the bottom and as close to the exact center of the bag as possible. Place any shoes in this area as well. This creates a solid, balanced base in which to build on with your other items. Having the heavier items on the bottom creates a safer load to carry because a top-heavy bag creates a hazard for the person carrying it.
Stand the bag straight up and fill the middle with your clothing. Roll up clothing rather than folding it as this takes up the least amount of room. Tuck smaller items such as socks and underwear into the side of the bag to fill the "cracks" left behind. Don't simply throw any items in the bag as that will waste space.
Store small items in plastic bags and put them on top of the bag as the last layer before sealing the bag. If you have any food or water, put them on the very top as they will be worthless if you can't access them in a hurry.
