The first thing that I do when I'm planning a trip is check the weather where I'm going. Which...in July is rather pointless as it is hot no matter where you go. It hit 100 in New York today. I'm going out on a limb to say it will be super hot.
The next is check with the airline (if applicable) to see what their specific restrictions are for carry-ons. I do NOT like checking bags. It's expensive, it's time consuming when you are taking connecting flights, and risky. I don't always trust TSA agents who check my bags when I cannot see them do it.
I'm flying Delta, so http://www.delta.com/content/www/en_US/traveling-with-us/baggage/during-your-trip/carry-on.html
Now I decide what to pack based on my itinerary. The key to packing efficiently is making the most of each piece that you pack. I always try to find a day outfit that transitions quickly and easily into night.
Friday - We arrive in Nashville at 11pm local time, pick up our rental car, and head off to the hotel to pass out. Room service, anyone?
Saturday - Sight seeing! We are meeting up with some friends and seeing what Music City has to offer. You can't come all this way without seeing the touristy things! AND SHOPPING. Opry Mills Mall and Green Hills. Tory Burch... Off Saks 5th... Etc.
Sunday - More touristy things. Opryland, Country Music Hall of Fame, etc.
Monday - VANDERBILT! Meeting at 9am and then touring the campus. ADPi house is definitely on my list
The act of packing. Obviously you want to put your soap and liquidy items in a ziploc to make TSA a breeze and don't end up with conditioner in your shoes...been there.
This spreadsheet SAVED me. Instead of standing in front of my closet thinking, "I'll just pack one of everything" then ending up having to go shopping when I get there because nothing goes together.
I hope that this helped you think of your next vacation as a streamlined procedure rather than complete chaos.
And see my Levo mentor, Carly Heitlinger's blog for a GREAT carry on packing tutorial! http://www.thecollegeprepster.com/2013/07/what-to-pack-in-carry-on-bag.html