Airports can be incredibly confusing.  Here’s what to expect at airports when traveling to make your trip smooth and enjoyable.
What to wear on the plane
Before you even get to an airport, your clothes will be a big factor in your enjoyment of the flight.
Dress comfortably, simply, and light.  The temperature on the plane will be unpredictable.  If the plane is full, it can be very hot and unpleasant to be overdressed.  If everyone has turned on their AC to the max, it can be cold and the extra layer your brought in your carry-on will be very welcome.  Try not to wear something that is difficult to put on or take off in a cramped space.
Airport security in the US will often require you to remove your shoes during the security screening.  Other countries generally don’t make this check.  If you expect to take off your shoes, try and wear the ones that come on and off easily.
International flights and/or long flights (6 hours+) will often provide a little blanket and pillow for extra comfort, but it may be easier to bring your own beforehand.
Packing
Luggage fees are the absolute worst things in the world, and I want to help you avoid them.  Check your airline for their specific checked bag and carry-on requirements as they differ airline to airline.  Don’t get surprised at the airport when you arrive to find out they want to charge you $50 each way for having a bag that is oversize/weight.
Don’t tiptoe the line and pack your bag right to max capacity.  If someone wants to ruin your day because you’re a pound over the limit, they will.  Give yourself some wiggle room.
In general, carry on bags for domestic US flights should be 22″ long x 14″ wide x 9″ high.  45 linear inches (height + width + length).  There is generally no weight limit for carry-on bags if they fit this size.
Checked bags will generally have weight limits of 50 pounds or you have to pay a fee.
Flights outside of the United States will have smaller bag limits!  I’m sure lots of people get caught in this trap: they take their 45 pound bag internationally only to find out that the limits there are 40 pounds.  In general, international travel will limit checked bags to 40 pounds and carry-on bags from 7 to 10 kg.  The dimensions on carry-on bags will also be smaller but vary by airline.  Check your specific international airline for the exact sizes.
There’s always someone at the airport re-packing their bags after throwing items away just to avoid paying another $100 in fees.  Don’t be that person!
Packing for international flights
Bring a pen to fill out immigration and customs forms on the plane.  This saves quite a bit of time while the rest of the plane is trying to use the 5 community pens they have.
Getting there
Arrive 1 hour before departure for domestic flights and 2 hours before departure for international flights, no matter what the airport.  The time listed on your ticket is the departure time, not the boarding time unless specifically mentioned.  Do not try to show up at the gate 15 minutes before departure!  Many airlines will not let you board.
There is very little reason not to show up early.
Checking In
Airlines now offer e-check-in 24 hours before the flight.  At this time you can pick your seats before the rest of the people and let the airline know that you do intend to be there so if something goes wrong they can wait for you.
Check in is also available at the airport either at the service desks or automated kiosks.  We prefer the automated versions because they are fast and you don’t have to wait in line.  But, sometimes they don’t work and/or direct you to the service desk anyway.
Liquids, Weapons, Food
In general, you can’t fly with liquid containers holding more than 3 oz / 100ml.  Keep your liquids inside of a 1 quart ziplock baggie.  They will make you throw them away if you don’t.
“Weapons” might sound silly, but that little pocket knife you have on your key-chain isn’t going to be allowed on board.  Other things you might not think are weapons, but security will surely protest.
Food in general is OK, but sometimes customs will want you to declare any food items that you have.  We try to make it easy and just not bring food across, but if you are bringing a special treat just declare it when they ask you to and it should be fine.
Have your ticket and ID/passport ready at all times
People all around the airport are going to ask to see these all the time.  Don’t put them deep into your luggage until you’re actually on the plane.
Tickets are available in electronic form on your smartphone, reducing the amount of papers you have to carry.  Passbook on the iPhone is one such app that takes airline tickets.
Going through security
Everyone hates going through security.  Don’t try to fight it, they will just make your life hell.  Be calm and quick and they won’t bother you at all.
In general:

• take out liquids
• take out laptops/kindles
• take everything out of your pockets
• take off your sweatshirt/coat
• Put the above items into two trays: one for the laptop and everything else in the other
• US: take off shoes
• Stay with your items until they go into the machine, then walk through the scanner.  Pay attention to the person on the other side if they want to scan or frisk you.

Make sure you pack liquids/laptops in easy to get to locations.  Don’t bury them deep inside the suitcase.
Don’t be lazy about these.  If you leave your liquids inside the suitcase, they will want to search your entire suitcase.  That tightly packed spring will uncoil and become a huge hassle, costing you time and happiness.
Finding your gate
See the gate number on the ticket?  That’s a lie.  Gates change all the time.  There are monitors all over the airports telling you the up-to-date information.  Never trust the gate number on the ticket.
If you are sitting at your gate and no one else is there and it’s getting close to your departure time, your gate has moved.  Find a monitor with gate information and listen to the announcements to get the right gate.
When you are at what you think is the right gate, verify it by looking for your flight on the monitor in front of the loading doors.  It may have changed while you were walking.
I cannot stress it enough: the gate will change randomly without warning.  Keep tabs that it’s still correct.
Getting to your gate
After getting to the airport and through security, it’s time to relax… WRONG!  Find out where your gate is from a monitor and relax nearby.  Some airports and ridiculously large.  So large that you have to take buses or subways between them.  You might think there’s enough time, only to realize too late that the airport is massive.  If you are not familiar with the airport, get to the gate first just to make sure you can do it on time.
During the flight (international)
During an international flight, the flight attendants will hand out immigration and customs cards.  Fill these out on the plane to get through customs much quicker.
Dealing with Immigration
If this was an international flight, you will have to go through Immigration (the process of entering a foreign country).
They are going to be checking your passport as well as immunization history.  Check the country you are traveling to and make sure you have enough pages in your passport as well as the correct immunizations.  The vaccine for Yellow Fever is a very common request, and you will not be allowed back into the US if you do not have it!
In general, Immigration will want to know:

• where you are from
• why you are here
• how long are you staying – in many cases, proof of onward travel is required (they want to know you will leave at some point)
• what location you are visiting and/or address of hotel

Dealing with Customs
If this was an international flight, you will have to go through Customs (the process of taking items in and out of a foreign country).
Customs varies widely from country to country, but in general it’s best not to have the following:

• $10,000 USD or equivalent in cash
• goods intended for sale in that country
• drugs
• food products
• plants
• animals

Yes, even that prescription you have may be illegal and subject to be seized.  Check the laws with each country before going.  Most people will not have a problem.
Be aware of the time when you land
Many flights will arrive at late or super early hours.  Somethings that are normally available might be closed, like buses or currency exchanges.  Plan accordingly.
Dealing with Taxis
Taxis from airports are going to be higher priced than you’ll find elsewhere, but I’ve rarely had problems with them being downright sleazy.  You can do a little shopping around within the airport, but if you haven’t found a better deal after 10 minutes of searching then it’s probably not worth looking further.  Some airports have fantastic public transportation to begin with.  Map out your route from the airport beforehand.
Getting cash
Most airports will have many ATM’s as well as currency exchanges.  However, make sure the currency exchange is open when you are arriving.  If you are unsure, get money changed beforehand in your originating airport.  Travel with a credit card for most purchases and a debit card for ATM withdrawals to avoid the credit advance fees.
That’s it!
Most airports are pretty much the same in how they do things.  It’s important to give yourself enough time to avoid missing the flight and potentially ruining your vacation.  By having done a little legwork beforehand, you can have a smooth experience and a great trip.