Your mandolin will stay in better condition (thus increasing its resale value when you want to upgrade) if you take some simple steps to care for it.
If your mandolin did not come with a case, consider buying one for it. Cases come in all types and price ranges. Chip board cases are relatively inexpensive, but also offer relatively little protection. If you push on the sides of a chip board case, they will move in and out. If you have this type of case already, you might want to consider putting a (thin) cloth, both underneath and on top of your mandolin before you store it. This will give the instrument an extra layer of protection. You don't want to put such a heavy layer of protection in there, that you can't close the case easily (like an overstuffed piece of luggage). That could actually damage your mandolin, because it would compress it.
Gig bags are fabric bags sewn in the shape of the instrument, and they're usually a little more expensive than chip board cases. Most of them are made so that you have the option of carrying the instrument on your back, like a back pack, or by carrying it in your hand. Gig bags are generously padded, so they offer somewhat more protection than a chip board case does. Like a backpack, they usually have numerous pockets where you can stash all of your mandolin accessories – books, strings, tuners, etc.
Hard-shell cases are the most expensive, and most mid- to high- end instruments come with one with the manufacturer’s name on it. The exterior of a hard-shell case looks like luggage. The interiors are cut out to specifically so that the instrument, whether A-Style or F-Style, will fit snugly, and then there is padding all around the instrument. They have small compartments inside for storing some accessories, and are much sturdier than gig bags. When driving to and from practices, I set my hard shell case on end in the back seat and then run the shoulder harness across the front of the case, through the handle, and then fasten the seat belt. That way, if another driver hits me, my mandolin doesn't go flying around the car.
Hard shell cases are NOT rated for airline travel, but I've not found this to be a problem when flying because mandolins are small enough to fit in the overhead compartment anyway. I just take mine on the plane as my carry-on luggage. But if you wanted to send a mandolin as "baggage" on an airplane, you would first need to purchase a very expensive type of hard shell case that is specifically designed to withstand that kind of abuse.
Before you play your mandolin, you might want to wash your hands – particularly the fingertips. When you are done practicing your mandolin, you should rub the strings down with an old cloth. Some people also like to use a product such as "Fast Fret", which is a cleaner that is rubbed directly on the strings. Cleaning the strings off helps to remove skin oil, perspiration and bits of dead skin that flake off of your fingertips as you play. Doing it after every practice session will help to prolong the life of your strings – and they will sound better, longer.
Extreme temperature and humidity changes can literally cause any musical instrument to split apart. You don't want to keep your mandolin in a place that wouldn't be comfortable for you to stay in for a long time – like your attic, the basement, or sitting right next to the heat register in your living room. You also don't want to leave a mandolin sitting in your car, in any type of weather.
If you don't have whole-house humidity control, you may also want to consider buying a humidifier to keep in your mandolin case. These are longer rubber tubes that have a sponge inside. The tube is soaked in water, and then squeezed to remove the excess. The tube is then placed inside the case, in a place where it does not push against the mandolin when the lid is closed. Depending on the weather, the humidifier should be refreshed about once a week.
Depending on how valuable an instrument you own, you may want to consider buying insurance on it. Most homeowner's and renter's insurance policies provide very limited coverage on musical instruments ($250. - $500). That would be more than adequate to cover a beginner instrument, but, if you are fortunate enough to own a high end instrument, then you might want to investigate the possibility of adding a special rider to your homeowner's/renter's policy, to insure it. Most insurance companies would require that you first have the instrument appraised by a qualified appraiser, before they would give you an estimate. But if you just bought it, and you have the receipt showing what you paid for it, then they should be able to give you an idea of how much it will cost.
Last but not least – please recognize that, no matter how much you might baby your instrument, there are things that can happen to it that are completely out of your control. I restrung my F-Style mandolin one day and, while I was polishing it (more about that in the Restringing section), I noticed a tiny crack in the varnish – right between the neck and the body. This particular instrument happened to have a manufacturer's warranty, so I took it to the authorized repair center that the manufacturer required. I was not concerned about the appearance of the crack, but I was concerned that I could cause further damage to the instrument by playing it. The luthier at the repair center sent it back to the manufacturer for their opinion. The manufacturer determined that the instrument would be damaged even more if I continued to play it. The crack had happened not because of neglect on my part, but because the tension of the strings on every mandolin pulls the neck toward the front – like a hunter's bow. In this case, there was a weakness in the wood that they had not detected in the building process, and that tension caused the neck to separate from the body of the mandolin. So the tiny crack that I could see now, was the beginning of what was going to be a very big problem.
And this brings up something that you should know, about how luthiers make their livings. They charge by the hour, and a good one is worth every penny. But if their rate is $40 an hour, and the repair you need will take them 20 hours to do, you will get an $800 repair bill. That would make perfect sense if you paid $5,000 for your mandolin, or if you paid $200 for a vintage mandolin which, when the repairs were made to it, would then be worth $2000. In my case, however, I had not paid a lot of money for the mandolin to begin with. But it did have a warranty, so rather than repairing it, the manufacturer "totaled" my mandolin and sent me a brand new one – because it was cheaper for them to do that than repair it. If the instrument did not have a warranty, I would have had the option of paying more to repair it than what I had originally paid for it, or throwing it away and buying a new one.