Buy a variety of sizes of zipper storage bags. From snack size to jumbo bags large enough to hold a winter parka, there are bags to suit your needs. Those that close with a slider tend to stay closed in your suitcase better than the press and zip types. Buy sturdy bags for shoes and belts. You can scrimp on costs for the smaller bags and buy generic or store brand products.
Lay out your clothing on a flat surface, such as a bed or table. Arrange your items by day and lay out everything you will wear together, from underwear, socks, jewelry to your top, pants or skirt. It is a good travel practice to minimize the amount of jewelry and shoes you bring along, so if you are duplicating certain items during your trip, lay them out with the first outfit with which you will wear them.
Insert the entire outfit, accessories and all, into a large zipper bag. For children, you should be able to fit an entire ensemble in a gallon-size freezer bag. For adults or heavy items, you need a larger zipper bag, such as those commonly sold to accommodate sweaters or sports items. Try not to worry about expense, as the bags can be reused each time you travel. Small loose items, such as jewelry can be zipped inside snack sized bags before being put in the larger outfit bag.
Apply pressure to the bag to reduce the amount of air you are packing. Close the bag almost all of the way, then squeeze out as much air as you can and zip it quickly to trap the air on the outside of the bag. This leaves you with a neat flat package that can be stacked in the suitcase in an organized fashion.
Seal toiletries in individual zipper storage bags. Not only does this keep them organized, it prevents messy leaks in your suitcase and protects your clothing. Also, the smaller toiletries bags are easy to grab when running them through security clearance at the airport.
