The most common reason is late check-ins. When you check in late for your flight, you are increasing the odds that your checked luggage won’t get on the plane in time. Those bags have to be scanned and sent through a complicated conveyor belt system before they get loaded onto a cart, transported, and loaded onto the plane.

If your bags were delayed because you checked in late for your flight, most airlines won’t cover the expense of delivering the bags to you. You’ll usually have to pick them up yourself.

Changing flights after checking in your bags could also nullify your right to have your bags delivered to you.

In some cases, it’s nearly impossible for the airline to deliver your bags in a timely manner. If you’re staying in a hotel that’s hundreds of miles away from the airport, for example the airline will probably resort to sending your bag via FedEx (within the US, anyway). That could mean you’ll have to survive an extra day or two without your bag.

In short, yes, if the airline temporarily loses your baggage,  they will most likely deliver your bags to you at no expense. However, if checking in late was the reason why your bags were delayed, don’t expect the airline to deliver.

Although you’re allowed to carry-on bags with dimensions up to 22″ x 9″ x 14″, some bags are simply too big to fit in the overhead compartments of CRJs and other small planes. That’s why some airlines such as Continental let you to skycheck your carry-on luggage.

Here’s how it works: When you’re about to board the plane, the airline employee who’s scanning the boarding passes will have a look at your bag. If it’s clear that your bag won’t fit in an overhead compartment, he’ll attach a tag to your bag. You leave your bag at the end of the jet-way (or in a trolley on the tarmac), where a baggage handler will take it and load it onto the plane.

When you’re leaving the plane, you don’t have to go down to baggage claim to get your skychecked bag. You just wait by the aircraft until someone brings your bag out to you.

Skychecked baggage still needs to follow carry-on restrictions. The bag shouldn’t be too large or overweight. If an airline employee thinks your bag is too big or heavy, he might make you check it. This is a hassle you don’t want to go through, and you especially don’t want this to happen if you’re already checking in some bags. You’re allowed to check only so many bags for free; anything extra gets you slapped with excess baggage fees.

For a hassle-free skychecking experience, make sure your bag’s dimensions are no more than 51 linear inches (length + width + height). Also, make sure the bag is within the airline’s carry-on weight restrictions. Domestic airlines like Continental will often let you take as much as 40 pounds in your carry-on, but international airlines tend to be less generous.

And that’s really all there is to skycheck luggage.

Some travelers are pretty good about attaching a tag with contact information to one of the bag’s handles. That’s good, but those tags can be torn off. Stuff like that happens. That’s why you should put the same information inside your bag as well, along with a print-out of your itinerary. That way if the baggage belt system manages to eat the tags and an airline employee is forced to search your bag, they’ll find out who the bag belongs to and where it needs to go.

Some people are afraid to do this because they don’t want bad guys finding this information. I understand their concern, but the odds of something like this happening are remote. I don’t have official numbers to back me up, but I feel comfortable saying that your odds of losing your bag forever by NOT including this information are better than the odds of a thief rifling through your bag and targeting your house while you’re away.

If putting contact information in your bag still makes you uneasy, please include your name and cell phone number at the very minimum! Alternatively, you could use a business address.

I’ve seen a few John Doe bags: They had no identification, no clues for us to go by. Eventually we had no choice but to ship such bags to our central baggage office. I assume they were eventually sold at auctions.

Don’t let that happen to your bag. Put contact information on both the inside and outside of your bag.

That’s not going to be a major issue for too many people, since Southwest claims that only two percent of their passengers check a third bag. Plus, the excess baggage fee is only $25. Compare that to other airlines that sometimes charge as much as $200 in excess baggage fees!

Just make sure the bag isn’t over 100 pounds, because Southwest Airlines won’t accept anything that heavy. Fuel isn’t getting any cheaper these days.

You can thank the lovely people who were planning to blow up airplanes using a cocktail of peroxide explosives and Lucozade, a British sports drink. Liquids were banned shortly after London police arrested 24 suspects on the night of August 9th, 2006.

The bottles of Lucozade were going to be filled with the original drink. However, the would-be terrorists planned to add fake bottoms filled with some explosive gel or liquid dyed red to match the rest of the drink. The explosive substance they were planning to use would be very sensitive to heat and friction, and could have been ignited with an electrical charge. Some reports said plotters were going to set the things off with iPods.

Chaos followed. People were immediately banned from taking liquids and gels on the plane, even if it was just a bottle of water bought in a secured area. TSA spent a lot of time throwing away drinks, cosmetics, toothpaste, hairspray, and all the other different essentials people take with them when they travel. The only liquid they allowed was baby formula.

Although the ban on liquids eventually relaxed, we now have to follow the infamous 3-1-1 rule when it comes to taking liquids on the plane. You are only allowed to take as many 3-oz bottles of liquids that will fit inside a 1-quart zip lock bag.

So the next time your zip lock bag comes open at the security checkpoint and spills out all your teeny bottles of soap, lotion, and shampoo, you can thank those terrorists for making airline travel just a little bit harder for the rest of us.