Picking your Coffers and Chests

The Tools

 * Thief's Tools: These are the cheapest tools. They're not considered "the best" though, although I've read that some people think they are the best after level 60. These can be crafted with Goldsmithing, Smithing, and Woodworking.
 * Living Key: These cost more than Thief's Tools, but are somewhat better supposedly. Rumored to have a higher chance to getting a Mimic. Can be crafted with Alchemy skill.
 * Skeleton Key: These can be quite expensive, but they seem to be the best. They're still not 100% though. Crafted with Bonecraft and Alchemy.

The Picking
The basics of it are that you go to the zone, find the coffer/chest, trade your tool-of-choice to the coffer/chest, and hopefully it works! If it doesn't work, one of 3 things can happen:


 * Nothing, it just says you failed to open the coffer/chest and it continues to sit there waiting for you to try again!
 * You get weakened. It's like a bad raise sickness that can last for a few minutes to an full hour (or longer...?). During this time you can not try to pick the coffer/chest again, you have to wait for it to wear. The only way to get rid of it quickly (I believe) is to die. Some people bring a Scroll of Instant Warp and warp to their Mog House to change jobs for this purpose.
 * A Mimic spawns!! This is not good and you probablly want to get out of there as fast as you can, or die. More on Mimics can be found below.

Failure?
There seems to be a strong correlation between the success of a pick and your level to that of the mobs in the zone. Basically, to have a good success rate, you want to be near the level of the mobs in the zone. You will probably fail over and over again if you were a 20 thief picking coffers in Castle Oztroja. But as a 58 thief you should have a much better success rate.

Once you've failed a picking, you will lose the tool that you used. So it's possible to go through a stack of tools and not succeed.

Chests as a general rule of thumb tend to be a bit easier to pick than coffers.

Mimics only spawn from failed coffer picks. They can last in a zone for quite a bit of time.

Additional Notes
After a coffer/chest has been picked, it can take for 15-45 minutes to respawn.

If you get a critical failure (i.e. Mimics from failed coffer picks), the coffer/chest can respawn immediately (usually between 5 to 15 minutes).

Here's my take on keys as a level 52 thief. I went to Monastic Cavern and used 4 Thief's Tools and 4 Living Keys on the coffer for my AF hands. I failed and even got weakened at one point. After leveling to 55 we went to get my AF in Castle Oztroja. I decided to use the expensive Skeleton Keys. After finding the coffer, I failed 3 times and succeeded on the 4th. Whether that was because I was higher level, the zone changed, or I used Skeleton Keys I don't know (I'm sure it was a combo of all three...).

You can also get 5 free Skeleton Keys (one every 30 minutes) from the quested item: Key Ring Belt. It's an easy quest in bastok called A Bait and Switch. Choose Lucky Charm at be beginning, and you will be able to get the Key Ring Belt. Quest can be repeated once a week if you drop the Key Ring Belt before you activate the quest again.

If you possess the coffer key for that dungeon anywhere in your inventory (mog house, mog locker, inventory, etc.), and you try to use any type of Thief's Tools, you will fail, so use the actual coffer key if you have it! (This is from personal experience, I used 4 Thief's Tools and I failed, weakened, mimic, failed)


 * Notes from a lv. 75 THF*

Post 60 Thief's Tools are by far the best.

As for possession of the coffer key while trying to pick, absolutely not true. It doesn't make any difference. While wiser to use the coffer key (guaranteed versus random) it won't prevent you from picking it. Remember, most of everything a thief does is rather random, much like using the /random function in game. Testing somethign 4 times is never an adequate show of results.

More on Mimics

 * They will instantly do an AOE poison that eats your HP. It will also cancel your Utsusemi.  They can also do an AOE Stun or AOE Paralyze attack but not as frequently.
 * You can't run from them - they have an ability that draws you in. It basically transports you from your running-away to right in front of it again. Even Flee can't stop him.
 * Recommended escape: Hit Perfect Dodge and use a Scroll of Instant Warp (or some other Warp item). Run as fast as you can from your Home Point to your Mog House to get rid of lingering poison effects.
 * A party of level 60's should be able to kill it. It is possible for a 75 THF/NIN with antidotes and status bolts to solo most mimics in zones, depending on where that mimic is located.
 * If it's killed, a coffer key will drop - no ifs ands or buts. It will drop.
 * When it is killed, a coffer will spawn, sometimes (always?) in the same location.

See the Treasure Chest/Coffer Guide if you're interested in what they contain and where they are.

Thanks to Rayel for great information!
 * See his post at: http://ffxi.allakhazam.com/db/jobs.html?fjob=10&mid=1086187627165365190&num=150