I see others have provided their checklists, but FWIW I'll throw mine in:
I do this regularly. It has gotten a bit more difficult with the addition of checked baggage screening by the TSA, but here are my tips (worth every cent you paid for them):
1) Carry documentation: Get a paper copy of the relevant airline's "firearms in checked baggage" policy, as well as the TSA policies, and carry it on your person when checking in. If there is a partner airline involved and you will be boarding the initial leg (either direction) on that airline, carry their rules as well. You can see the TSA policies at:
TSA | Transportation Security Administration I used to carry the FAA policies, but haven't found them on the web lately. You can usually print the airline's policy from their website. If the policy appears too brief in the website, it is worth inquiring at a ticket counter and asking them to print their full policy out of the ticketing system. This is too late when you are already checking in, particularly if you are getting hassled by the ticket agent. Plan ahead and get this copy during another trip through the airport.
2) Avoid Northwest Airlines, especially in Detroit. Recent personal experience indicates that they are incompetent, dangerous, and don't know their own rules.
3) Ammo: Per the FAA/TSA regulations (they were more complete on the old FAA website which I can no longer find), you may carry a small amount of ammunition "for personal use". The old FAA site specified this as 11 Lbs, but the new TSA site does not specify. The FAA site required that the ammo be in either "factory packaging" or magazines. The new TSA site requires that ammo be in "fiber, wood or metal boxes, or other packaging specifically designed to carry small amounts of ammunition." Most airlines have based their written rules on the old FAA rules (including acceptance of ammo in magazines, as long as the mag is not in the pistol). The TSA employees seem to interpret the new rules as allowing only factory-type ammo boxes. I have NEVER seen a rule requiring that the ammo be in a separate case from the firearm, but many ticket agents (and some TSA employees) believe that it is a rule. Some airlines specifically note in their rules that it IS allowed.
4) Cases: The rules require that the firearm be in a locked hard-sided container, which could either be hard-sided luggage or a hard gun case within soft-sided luggage. I can't remember if it was in the old FAA rules or in a specific airline's, but there used to be a rule that the soft-sided luggage containing the hard case must also be locked.
5) At the counter: Most obvious thing is to make sure it is unloaded BEFORE you get to the airport. When you get to the ticket counter (must be an inside counter rather than remote or curbside check-in), first thing is to discretely ask for a declaration card for firearms in checked baggage. They may screw up and hand you a form for carrying firearms on your person. Make sure that it is a small card indicating that it is for checked baggage. Some airlines (e.g., Northwest) require the ticket agent to initial the card. Some agents have interpreted this to mean that they must physically check themselves that the firearm is unloaded (handling the firearm). I have NEVER seen airline rules which allow this, and most (including Northwest) specifically prohibit their employees from handling the firearms. If someone insists on handling the firearms, I take out the printed rules and ask them to read them and call in their supervisor if there is still any question. If they get insistent, I usually flip over my license and show them my NRA instructor certification card, and ask them what certification they have for firearms training. Always remain polite and composed, and start writing down details and names if you are getting hassled (it seems to remind them that they may be held accountable). Some agents will insist that the ammo must be in a separate suitcase (false, and I ask them to please show me a written rule), and some will also insist that there can be no more than one firearm per case (also false, and usually specified to the contrary in the airline's policy). According to the old FAA rules, the declaration card was specifically required to be placed within the "locked hard-sided case" with the firearm. It also specified that there be no external indication on the case or suitcase that it contained a firearm. While the TSA rules are much more brief, the airline rules are usually based on the FAA rules. I always refuse to allow them to put the tag on the outside (obvious "STEAL ME" sign), and sometimes have to argue with them about putting it within the hard gun case.
6) After check-in: Depending on the airport, you will likely be required to accompany the suitcase to the TSA screening (be sure to budget an extra 15 minutes for this step when arriving at the airport). They will usually have you remain just outside of their screening area, and have you hand them the keys to your suitcase and gun case when they get to it. If the airport is set up where you carry your checked bags from the ticket counter to the TSA station, be sure to inform the TSA that your case contains declared firearms. Some TSA people claim that the shift supervisor must be present when they pass a firearm through, so there may be extra delay while they get him out of the donut shop. This delay is usually spent with all the TSA people crowding around the X-ray screen, trying to guess where the firearm is in the case.
7) Carry printouts of the relevant CCW laws at all destinations. Many LEOs are ignorant of the actual laws of their own state, so it is good to have your own reference.
8) Carry a copy of the sales receipt for your firearm with you, particularly if traveling into borderline communist territory.