Items you should never
put in a checked bag

There are many items that are not hazardous or prohibited that you can carry either in checked baggage, in carry-on baggage, or on your person. However, if it is an item that is hard to replace, very expensive, or necessary for your health and well being, then you should never put it in checked baggage:

Money related items: Cash, credit cards, travelers checks, blank checkbooks, securities, and anything else that has monetary value or should either be on your person or in your carry-on baggage. If you lose money-related items in your checked baggage, airlines are not obligated to compensate you.

Jewelry: Necklaces, rings, diamonds, other precious stones, gold, silver, other precious metals, expensive watches, and other small and valuable items like these should also stay out of checked baggage. Like the situation with money, the airline isn't obligated to compensate you for the loss.

Laptops and electronic devices: Laptops, cell phones, and other small personal electronic devices should remain on your person or in your carry-on bags.

Medically related items: Prescription medication, other medications, and other medical items should remain with you in the cabin.

Other items: If it is small and valuable, or if replacing it would be difficult or expensive, then keep it with you in the cabin. Examples include passports and other identification, keys; eyeglasses or sunglasses; photos, exposed film, tickets, art, boarding passes, travel vouchers, mail, financial records, business documents, manuscripts, heirlooms, collectible items, favorite toys, portable data storage devices like flash drives, and software. If you have something else that is small and that you don't want to lose, keep it with you.

Last minute bag check:
Sometimes on full or nearly full flights, the airplane may run out of room, and you may be forced to check your carry-on bag at the gate. You should pack your carry-on bag in a manner so that if it must be gate-checked, you can quickly remove the fragile, valuable and critical items and have them in a smaller bag that you can fit under a seat.

Related resources
Baggage Basics for Checked and Carry-on Items
Carry-on Baggage Advice
Top 10 Baggage Tips
Hazardous and Prohibited Baggage Items
Duty free liquid and gel items
Dealing with security
Laptop security