Most people overpack for the same reason: they pack for every "what if". This edit takes ten minutes and works for any bag.
1. Lay everything out
Put everything you plan to bring on the bed. Seeing it in one pile usually shows duplicates fast.
2. Plan outfits, not items
Write down one outfit per day, plus one spare. If a piece isn't part of at least two outfits, it's a candidate to leave behind.
3. Cut a third
Take out a third of the pile, starting with "just in case" items. Anything you can buy or borrow at your destination can come out too.
4. Pick a color story
Choose two or three colors that go together so every top works with every bottom. This is the single biggest reason people pack fewer clothes.
5. Shoes: three at most
Wear the bulkiest pair on the plane. Pack one more pair, and a third only if the trip needs it.
6. Pack in groups
Put like with like (tops, bottoms, underwear, small things) so you can see what you have and stop adding more. Packing cubes are one way to do this; a plain zip bag works too.
7. Weigh and check
Weigh your bag at home and check your airline's size and weight limits on its own site, since they differ by airline and fare.
8. Re-edit on the way back
Note what you didn't wear. Next time, leave it home.
Related guides: 5-day carry-on packing list and 1-week carry-on packing list.
From Less2Lug: our 7-Piece Packing Cube Set splits a suitcase into clothes, underwear, shoes and small things (standard packing cubes, mesh panels so you can see inside). Free US shipping, tracked; delivery in 8-14 business days.