Before you start packing it is always a good idea to make a list so you know what to bring, it keeps everything a lot more organized and makes sure you won't forget anything important, such as medication. I suggest making the list a week or a couple days before your departure. This way, as time passes you might remember a few more things to add to your list that you forgot originally. When you go about your day, for example, you might pass by your closet and remember that you need to bring a hat and gloves cause it's going to be very cold.
Always pack light. You may be afraid you're going to forget something, so you say to yourself: "It's always better to be prepared!" Well, yes, it is better to be prepared, but you don't want to end up bringing extra stuff that youre never going to need. You don't want to be lugging around a huge, extremely heavy suitcase just because "you want to be prepared", which is why you should only pack necessities. If worst comes to worse, you can buy whatever you need at the store when you get to your travel destination, it won't be the end of the world. Think to yourself while youre packing, is this item worth the extra weight of me carrying it around town? If not, put it back.
You've probably seen videos on Facebook or Twitter titled "tips for packing", and they usually all suggest rolling your clothes instead of folding them when you're packing, and there's a reason for that. Rolling your clothes gives a lot of extra room for other things you may need. I know in the first tip I said to pack light, so this rule still applies, its just a lot easier to keep your clothes neat and more compact when they are rolled up.
Bags, such as ziplock bags or small plastic shopping bags, are a good way to keep your stuff organized and clean. Whenever I travel, I put my shoes in plastic shopping bags to make sure any excess dirt doesn't get on my clean clothes. I use ziplock bags to put small items like makeup products, q tips, small shampoo/conditioner, headphones etc. to make sure they don't get lost. I'll usually put similar items together to make sure I have everything. For example, bath products go together like makeup products will go together.
There are usually weight limits on your check in bag, before you leave, check your airlines website for weight guidelines. Weigh your bag at home to make sure it doesn't go over the weight limit, an easy way to do this with an at home scale will be to first weigh yourself, then weigh yourself holding the suitcase and subtract your amount. This will give you peace of mind to make sure your bag isn't overweight, because if it is there is usually a very heft fee for overweight baggage. Another important tip, if your bag is very generic, is to decorate your bag with either a small scarf or ribbon of some kind. This way, when you are waiting to collect your baggage at baggage claim, it will be easy to spot so that you won't be pulling off other peoples luggage trying to figure out if it's yours or not.
On my trip to Rome, my check in bag never reached baggage claim because it was still back in the United States. I was without it for a week, which sucked, but luckily I packed most of my necessities in my carry on. I know it's not the best situation, but luggage does get lost sometimes, and it's not the end of the world. Make sure to pack the items you will need in your carry on, such as: medications, adapters/chargers, underwear/socks, change of clothes, toothpaste(travel sized)/toothbrush, travel sized shampoo/condtioner/soap, and whatever else you think is important to you that you won't be able to live without if lost.