Adding a 20kg piece of luggage to your low cost flight is reasonable enough, I booked an EasyJet flight and the 20kg luggage was only 26 Euro, 1.30 Euro per kilo. You can even add two more bags of 20kg each for another 26 Euro each. Fair price. But what happen if your bag is 23kg or 26kg? That's going to cost you, respectively, 50 or 74 Euro per bag. Meaning that every kilo over 20kg will cost you 6 Euro each! Instead, book a second 20kg suitcase, pay 52 Euro and now you can pack up to 40kg!
Long-haul flights often have different weight limits on different sectors and that can vary a lot. Example: Emirates basic allowance is 30kg, one or many bags. Most generous, and for USA and Canada flights, the limit goes up to two bags of 23kg each which makes it 46kg. Better yet, on South American routes, you can take two bags of up to 32kg each, a whopping 64kg! But if you go over those limits, expect to pay some serious money: Show up at Paris CDG to fly to Bangkok over the limit and the rate is 48 Euro per kg! 10kg over, both ways, could cost you more than your round-trip ticket! Buy the overage on-line when you book and you do get a 30% discount … that still works out to 33.60 Euro per kg!
At 0:15 there is an amazing shirt folding trick. It works, I got it on the first try!
So, before you choose the “roll it” or “cake it” packing method
Consider what goes in the bag and what stays home. First the obvious: Any 3-star on up hotel has hair dryers, you may have to ask if not in the room, but they have them. The lady's beauty case: How heavy is it? Really honest! Go through it and you will cut the weight in half, like leaving the heavy cologne bottle at home and packing the small perfume instead. Consider the small version of toiletries, or round up all the free samples you have accumulated or save the nearly empty bottles for trips and use the full ones for home. You can easily knock a kilo or two. Hard case or soft bag? Some hard cases are quite heavy while others are very light and they do protect your stuff better than the soft bags. In the winter, you don't have many alternatives for warm clothes and shoes. You would not pack sandals to go to Switzerland in January, you need warm shoes! But you can still shave off some weight by giving preference to silk (very light and yet warm) and wool (lighter than cotton) over cotton (heavy).
Bottom line: Consider the cost to pack it vs. the cost to buy it at destination. Especially liquids and spray cans. Even cotton t-shirts may not be worth their weight … beside they will probably cost more to laundry at the hotel than buying new ones across the street!