This article from Forbes hits the nail on the head. I have experienced every single one of these mistakes firsthand! Here is a recap, plus a little added commentary from yours truly: (for the full article click here)

1. Not booking your seat when you book your plane ticket! (yup, you will only do this once until you get stuck in the VERY LAST row by the bathrooms, with no reclining seat).
2. Dressing down for the flight. (It’s never a good day when you end up behind schedule and you have to attend a meeting in lululemon pants, and bed head…or having to get dressed in an airport bathroom. However, I NEVER check a bag for this reason as well)
3. Trusting that your flight will have WiFi. (I learned this the hard way when leaving the country for an extended vacation. I figured a 5 hour flight would be perfect for catching up on last-minute emails and setting my auto-responder. And then….you find out they have no wi-fi on the flight!) 
4. Wearing uncomfortable shoes. (There is a balance here! I am horrified when I end up sitting next to a man in flip-flops how takes his shoes off in-flight. Gag!)
5. Forgetting to check you plans before you land. (Use a travel app to stay organized, and double and triple check accommodations and travel times before hand or in-flight. As much as I hate to admit it, I have flown to the wrong city, I have forgotten to book a hotel, I have forgotten to book a car, and I have miscalculated travel time. Be prepared and double-check!)
6. Not bringing back-up cosmetics in your carry on. (If nothing else….deodorant?)
7. Not planning ahead to accommodate travel time from the airport. (see item 5 above)
8. Forgetting to bring an extra tote. (When traveling, if I have down time…I inevitably end up shopping. It seems the only reasonable thing to do, shop or eat! Throw in a little reusable bag or tote so you can carry extra items. You won’t believe how handy it is!)
9. Booking an aisle seat. (the article says it perfectly! must read!) 
10. Booking a window seat. (the article says it perfectly! must read!) 
11. Assuming you’ll get food on the plane. (There is nothing worse than running to catch a 3+ hour flight, and realizing that you are starving and your only food options is a small token bag of peanuts (delta now offers almonds), bring food with you!)
12. Not getting a jump on jet lag. (Think about how your flight times will correlate with meetings and time zone changes. Sometimes a little extra cash for a better flight time offers enough productivity to make it worth the extra spend!)
13. Suffering through coach. (Luckily if you travel extensively, and have status upgrades are truly a blessing. Once you have the chance to sit in Economy Comfort, Business Class or First Class, it’s tough to go back. Another justification is that more often than not you will be seated by other business travelers which offers a great networking opportunity!)
14. Not planning ahead if you want to bring wine home. (Great point! See #8 above, or leave room in your bag.  Utah has very strict laws regarding shipped wine, however, you CAN either put it in a checked bag, or many airports have wonderful wine stores where you can purchase it  in the terminal to carry on!) 
15. Thinking the gate agent will of assistance. (Basically, you are stuck with using your airline’s app, or call customer service. Anymore, the app is better to work with! )

Happy and Safe Travels!