I always make sure I have everything I need for survival or that would be hard to replace in my carry-on bag. (Medications, glasses, electronics, a few changes of underwear, etc.) If anything else gets lost I go shopping - it's happened a few times to me but it's not that big a deal, not worth stressing about.
For an extended trip, we'll often pack bags according to stages so we don't need to open them all to find what we want. eg if we'll be hiking at some point, all the hiking gear goes in a pack, if we'll need formal clothes, all the formal clothes go in a suit carrier bag, etc.
When the kids were little I always made sure they each had a bag they could manage by themselves, even if it wasn't big enough to hold all their stuff. For under-tens that was a schoolbag-sized case with wheels. Even toddlers can wear a tiny backpack. Then we had one big case for all their overflow. I found it easier managing one extra case than helping all of them with their bags.
I usually also pack a spare cheap squishy bag (the kind you use for storing blankets etc) in case we come home with more than we left with. Then lots of soft stuff (usually dirty laundry) can go in that.