This isn’t about how to find a cheap flight, but if you have a ultra basic no-frills essentials-only bottom-dollar economy ticket, and need to figure out how to pack and make the most of it. I’m most experienced with Flair Airlines and flying in Canada but I expect many airline procedures to be similar, ymmv. In the past you could often get away without measuring the bag but they have been getting a little stricter about it recently.

• Think beforehand what you actually will bring and what you plan to bring back. Will you pack food that will be gone before you return? An empty bottle? How many days of clothes do you need? How much in souvenirs will you get (if any?) Toiletries and extras, are there small things you will be able to pick up at minimal cost (such as soap bars that you get at a hotel)? Keep in mind that anything that you forgot to bring and end up buying at your destination will amount to more luggage on the return trip.
• Wear multiple layers of clothes to serve as next days changes. If you are going from a cold to warm climate, wearing several layers will save you from bringing the jacket you won’t need the whole trip. Warm to cold, stay in air conditioning, avoid overheating yourself, only wear it when baggage is being sized, either at the check in counter or the gate.
• Have your outermost layer be one with many pockets to put any extra stuff. Just don’t forget to take pocketed items out for scanning to have an easier time through security.
• A bottled drink at the airport is very expensive but if you are short on space you can use that instead of bringing an empty. (Important: Buy drinks after security!) Or pack one of those fillable water pouches which are very space efficient when empty.
• If you bring a bag that is slightly bigger than the size limits, make sure you can fold in or squeeze any parts that would stick out. A 30L backpack can fit when squeezed. Avoid packing your stuff too tightly and keep delicate stuff that might break when squished hard in your pockets until the bag is sized.
• Remember that once your bag is sized and you have a valid tag affixed, there will be less scrutiny to the actual size, so you’ll be able to pack more in your bag after validation. If you are near the back of the line when boarding, there is often even less scrutiny. Don’t go overboard with it, as the bag will still need to fit below the seat if overhead bin space is limited.
• If you have multiple people in your party not getting checked in at the same time, or someone you know well is dropping you off at the airport, place a lot of the contents you plan to bring in a separate plastic bag or whatever and leave it with your buddy, have your bag sized in the holder easily. Get the validation tag, then away from the counter, place the other contents back in the bag. This won’t work if you travel alone or in one group who are all checking in together, you don’t want to be marked as suspicious by leaving stuff unattended at the airport. Leaving stuff temporarily in your car at the airport parking, or in a nearby rental locker are alternative options, but give yourself extra time and research ahead.
• Once on the plane, if space permits you might be able to place your jacket and personal item in the overhead bins. Please be respectful to your flight attendants and fellow passengers, if you are asked to place your items under your seat you need to do that.