It's not your imagination:  Airport security lines really are getting longer. A nasty combination of more passengers and fewer screeners means that you could spend as much time in line as on board your flight.

The Transportation Security Administration's chief of operations admitted to The New York Times that "this is going to be a rough summer; there is no doubt about it," and advised people get to the airport "a little earlier than planned."

Or, a lot earlier than planned. Last month officials at Denver International Airport released a new recommendation that passengers arrive a full three hours before their flights. Even that extraordinary buffer wouldn't have been enough for the roughly 600 passengers at Charlotte Douglas Airport who missed their flights on March 25 after being forced to wait at TSA for more than three hours.

These incidents are far from being isolated. At airports across the country, passengers are facing historically long lines. While unforeseen security delays are a leading cause of missed flights, they're not the only cause. 

Here are four more mistakes to avoid.

1. Not accounting for travel time at the airport.

Your smart phone's mapping app will give you a remarkably accurate estimate of how long it takes to get to the airport. But getting there is only half the battle.

At major airports, it's not uncommon to need an additional half an hour to travel from the parking lot, rental car drop-off, or train station to your terminal. Remember to factor in this hidden travel time, and to double check which terminal your flight departs from.

2. Outsmarting yourself with a tight connection.

Every airport has an official minimum connection time (MCT), the least amount of time the average person needs to make a connection on a continuing flight based on factors such as airport layout, security, and whether you're connecting between domestic or international flights.

Booking sites show itineraries with layovers longer than the MCT, meaning that most missed connections are the result of unforeseen service delays. In these cases, airlines are legally required to help passengers reach their final destinations.

However, it's possible to have a connecting window shorter than the MCT if you book a series of separate one-way flights on your own, a technique some travelers use to hack together an itinerary for less than the price of a single ticket.

Booking separate flights can be a great value, but it requires accounting for additional logistical complexities. As a rule of thumb, give yourself at least 45 minutes between flights.

3. Not knowing the busiest travel times.

Most people know that the end-of-year holiday season is one of the worst times to travel. The day before Thanksgiving is as infamous for crowded airports as the day after Thanksgiving is for crowded shopping malls. Likewise, New Year's Eve and pretty much the entire week leading up to Christmas have well-deserved reputations for being potential travel nightmares.

As bad as things seem during the winter holidays, summer is the real peak travel season. Thirteen of the 15 busiest air travel days of the year fall between mid-June and early August, with passenger volume spiking on Fridays, Sundays, and Mondays.

Plan on needing more time to navigate through the masses during these windows, and if your schedule allows, consider flying on off-peak days. Not only will lines be shorter on Tuesdays, Wednesdays, and Saturdays, but ticket prices will be lowest.

4. Missing the check-in cutoff.

It's possible to miss your flight without ever getting stuck in a long security line. Airlines have specific rules for how far in advance passengers have to check in. The most common policy is 30 minutes for domestic flights and an hour for international flights, though some airlines have stricter requirements.

American Airlines, for instance, requires passengers on all domestic flights to arrive at least 45 minutes early, while other carriers close baggage check up to an hour in advance. To play it safe, always check in online ahead of time and avoid checking bags when possible.

Missing your flight is frustrating whether the blame lies with TSA or with you. 

Tolstoy wrote that "all happy families are alike; each unhappy family is unhappy in its own way." When it comes to air travel, there are many ways to miss a flight and be unhappy, but only one way to avoid this fate: show up on time. 

Err on the side of caution (and then some). Finding yourself with time to kill at the airport might not be ideal, but it's certainly better than finding you've missed your flight.