Begin your packing process days or even weeks before your departure date; this gives you time to draw up a complete list, as well as purchase any additional items you may need for your vacation. Creating a packing list is a surefire way to ensure that you never, never forget to bring something important with you. I tend to start packing a week in advance. It gives me time to add things as I think about them and also to rethink my choices and remove items.
We pack 3 to 4 weeks before a trip to make sure everything fits and we don't need anything that needs to be ordered. Then 2-3 days before we left, we unpacked and went through the checklist to make sure that nothing happened to be missed (I took the wrong contacts, forgot a charger, didn't take enough medication, etc. Pack in stages, pack things you won't need at home 2 weeks or more in advance, then add outfits 3-4 days before of the trip. Finally pack your toiletries, chargers and electronics the day before departure.
Make sure you pack things in the order you need them. Have a set of pajamas for everyone on top, so you can easily pick them up on your first night. Trying to carry very light luggage, in order to be able to travel only with hand luggage, is often a big mistake. I have a packing list that is always updated (especially right after a trip) and I buy most of the clothes in my closet to make them good travel clothes.
Make sure you focus on packing and not trying to solve other problems, it's very easy to think you did something when you're distracted. When it comes to things like bikinis and sunglasses, you have to be decisive and wear one or two pairs. I have this blank printable list that you can use, or this list of checklist styles that you can use to plan your packaging. Resealable bags are great for packing because you can use them to organize makeup, seal dirty underwear, make sure liquids don't spill all over your clothes, and much more.
However, unless you have a good reason to want to travel only with carry-on baggage, you should plan to pack a real (large) suitcase. Everything that has to do with your sleep routine (make-up remover, facial wash, etc.) or your morning routine (makeup, contact lenses, etc.) must go in your carry-on baggage. Pack your carry-on baggage with care, including the essentials you would need in case your airline loses your checked baggage. You have time to fine-tune your outfits, leave out unnecessary items, and also help others pack for their travels.
Many people wait until the last minute to pack their belongings in their suitcase, but others may have their things stored for months. I check my basic packing list about a month in advance to make sure I have the right items on hand. Overpacking leads to its own set of problems, such as excess baggage charges, damaged baggage, and absolutely no suitcase space for souvenirs or purchases abroad. There is no reason to be my messiest self, and before my next trip, I was going to become someone who would delight in suitcases, and a good time was guaranteed.
For me, clothes always seem to be the hardest thing to understand, so I always start there and choose a color palette.