I never fly with anything more than a carry-on suitcase that fits in the overhead bin and a computer case that fits under the seat.  However, if I ever do err on the side of bringing too much on a trip, it is probably for comfort items. I am a girl who likes her comfort.  For plane flights: I generally pack an inflatable neck pillow and earphones for the plane. I also bring a home-made picnic for very long flights to avoid eating poorly while traveling. For hotel stays: I bring the books I bought cheaply at the local thrift store (so I can justify dumping them anywhere at any time). I also like to bring granola bars (good for a midnight snack or quick breakfast) and a small nightlight (for the hotel bathroom). For all trips: I pack any medicines that I would want to have quick access to when far from home (bringing a few chewable Pepto tablets can be a life saver when you don’t speak the language).  I also pack a few plastic zip-top baggies of various sizes (they are extremely versatile and take no room at all).

My general rule of thumb for packing:
1)            Make sure you really need an item before it goes in your suitcase,
2)            Reconsider again whether you really need an item before it goes in your suitcase,
3)            If it is worth packing at all, bring enough for the entire length of your trip. Don’t plan on “buying more when I get there” for anything of real importance.

As a result of these self-imposed packing rules, I may pack lighter on the clothes and shoes, but heavier on the creature comforts and nice-to-haves.  I would be far more likely to eliminate an entire outfit than travel to far-flung destinations without my favorite tea to drink each morning. This has worked for me over 25+ years of traveling. Try it--you might like that comfortable feeling, too.