1. Use a travel planning checklist
A travel planning checklist may be one of the most overlooked tips. There is something cathartic about checking items off of a list, especially when you see your progress. In other words, whether you are planning a trip six months out or two days ahead, you can use this to check off what you need to get done ahead of time.
2. Create an itinerary
Keep an itinerary of the following:
Phone numbers of travel agents, rental car companies airlines, and hotels.
Flight numbers and times
Address(es) of hotels and any place you plan to visit.
When out of the country, the address and phone number of your country's embassy.
I personally keep a printed copy and one on my phone which I have emailed to myself and my parents in case of an emergency. I create the document as a google doc in order to ensure that it is available to me offline as well. Having all of this information written down will save you from wondering where you're supposed to be and how you're getting there.
3. Pack lightly and bring layers
I normally travel using only my carry-on luggage permitted by the airline. One of my petpeeves is waiting for the carousel to deliver my luggage from what seems like a portal from outer space.
In brief, the top packing tips are:
Make a list before you begin. It doesn't have to be overly detailed, but a general outline of what you'll need.
File fold your clothes and use packing cubes. You'll fit more into your suitcase.
Bring extra Ziploc bags and be amazed at how often they come in handy (think: leaky shampoo bottles, snacks, wet clothes, laundry, etc.).
When in doubt, choose a rolling suitcase over a shoulder bag. Your back will thank you.
Pare down shoes. They are heavy! Choose a few pairs that can multitask.
Always pack layered clothing. Chances are you'll be going between indoor and outdoor venues. You will want to vary your clothes by temperature possibly several times a day.
Consider complementary colors.