Remember that in addition to the bag of carry-on luggage that goes in the overhead bin, airlines also allow you to bring along a briefcase, purse, shopping bag or knapsack, provided that you can slide it under the seat in front of you. The precise size restrictions on the second bag vary from airline to airline. Delta Airlines, for example, specifies a maximum size of 9-by-14-by-22 inches (22.8-by-35.5-by-55.8 centimeters) [source: Delta Airlines]. But it doesn't say anything about weight. That's why packing expert Judith Gifford recommends packing some of your heavier items in the day bag, which will not only free up space in your suitcase, but will make it lighter to lift [source: Gifford].