Always be prepared for the eventuality that your checked luggage will be delayed or lost. Apply the “don’t put all your eggs in one basket” advice. Always pack a spare change of clothes in your carry-on bag as well as any essential items. For example, do not put important medication in your checked luggage. If your main checked bag is lost/delayed, you want to at least be able to get a set of fresh clothes on you the next day or two whilst you figure out what to do.
If you’re going on a very short trip and everything fits in one of those small carry-on suitcases, bring a personal item (purse, backpack, small bag etc..) for your spare clothing. Why? Because on overbooked flights, the overhead compartments can fill up very quickly. If this happens, the crew could force you to check your carry-on suitcase and have it shipped below deck. Once that suitcase is out of your hands, anything can happen to it. They will however always let you keep the “personal carry-on item”.
If you are travelling with a partner I suggest you each put some of your clothing in the other’s suitcase for better redundancy.
I travelled for over fifteen years without any single incident concerning my luggage. All of a sudden, in the span of two years, my checked suitcase failed to arrive at destination with me on two separate occasions. Luckily in both cases, the suitcase was found and returned to me a few days later.
Nowadays, processes are getting more and more automated so incidents involving lost luggage are surely dropping. However, humans are still involved in the process and at some point, mistakes will happen.