1. Not doing research
Before you travel to a new place, DO YOUR RESEARCH!!! What's the weather like? What are the locals like? Is it touristy? What are the chances someone there speaks your language? What are the fun things to do? Where are some good places to grab a bite to eat? What events are happening while you're there?
2. Budgeting incorrectly
Budgeting for a trip is necessary. Spend some time doing the math - how much will the flight cost? Or if you're driving- gas? Do you need to rent a car? Factor in gas for that too. Do you need train tickets? Where are you staying? Is it all-inclusive? Include food and alcohol. Will you be attending any events? What about excursions?
Having a good grip on the money you're about to spend is crucial. Be realistic with yourself about how much money you'll be spending. The worst thing you can do is short yourself and then have to cancel events or excursions because you don't have the money.
3. Sleeping it away
***ONE OF MY PET PEEVES!!!***
If you are spending hundreds or thousands of dollars to see a new place, ENJOY IT! If you're with your friends/family... ENJOY THE TIME TOGETHER! You have so much time to sleep when you get home. I'm not saying that you need to stay up 'til 4 am every night and wake up at the crack of dawn. But don't skip out on going to that cool breakfast place or going to a rooftop bar because you want to be well rested.
Typically vacations are 5-7 days-- sometimes even shorter. And most people don't have the money to just go back to any city they choose whenever they want, so enjoy it while you're there! You spent months saving money so you could experience this new place. Don't waste it away.
4. Being unprepared
For example- my sister and I went to Canada once without telling our bank, setting up our phones for international travel, or exchanging any money before we went. That was dumb. Very dumb. Don't be like us.
Another example- We went on a UTV tour of the desert and I forgot sunglasses (which are necessary to see considering sand flies into your eyes the entire ride).
This really just goes back to doing your research. Know what clothes you'll need (based on weather/excursions/events), what accessories you might need, set up your bank/phone info before you go... etc.
5. Not talking to locals
FIND A LOCAL, MAKE A FRIEND!!!!!
Learn about the culture you're in. Ask them what they recommend for fun and food. Get to know the area for what it really is and not just how it's portrayed to tourists. You'd be surprised at the things you learn.