Every camera is unique and that makes your decision so much more complicated but one thing unites them all (mostly): media storage. Even where a camera has internal memory you'll probably want to expand it. In Pit Talk: Do I briefly mentioned maintaining your storage media and here is where I expand on that.
IN CASE OF EMERGENCY STOP SHOOTING: It might happen that you accidentally delete images or see an error message and in that situation the best thing to do is stop. The key is to stop using the card so you don't overwrite data because it may still be possible to restore them.
CAMERA OFF / CARD OUT: There's this thing called, "voltage shock", that supposedly happens on removal of media storage while a camera is still on. I'd say always turn your camera off before taking your cards out as a precaution because your camera needs a little time to write all of the data to the media storage. On my camera there's a little green light that blinks when media storage is being accessed - that's a handy indicator to be aware of.
BEFORE YOU SWITCH OFF: A bit of science: after the sensor is exposed, image data will be processed in the camera buffer which temporarily stores the image information before it is written to the media storage. If you switch off your camera during this process, you’ll lose image data and could even find yourself with a system error or corrupted data.
REFORMAT (NOT ERASE): The unspoken rule is that the fewer times you add or remove data from your card the better. Reformatting the card after "dumping" (transferring) the images means a single erase cycle as opposed to multiple single erasures in-camera. As a ritual, every time you dump data then reformat your media storage. This wipes any data including images, videos and file names from the card. Lexar Memory media storage is bundled with access to Lexar® Image Rescue® Software, the software not only recovers photo and video files from any brand or type of memory card but also offers card reformatting and secure deletion features (so they're basically good as new), using any card reader and works with Mac or PC.
SAFELY REMOVE HARDWARE: After dumping content from a card, make sure you follow the proper procedure to eject the media storage safely before removing it from your card reader. If you yank media storage out while it's 'busy' then you run the risk of corrupting the data and you only need it to happen once for this to become a habit. There is no reason not to eject drives and hardware safely. Do this.
REFORMAT (IN-CAMERA): Yes, I said reformat it after dumping your data (as a way to refresh the file tables and restore the media storage afresh) but a good habit to cultivate is formatting your memory card in-camera afterwards. This ensures that the card is ready for use specifically in your camera (formatting it in-camera creates the file structure specific to your camera).
MULTIPLE CARDS: Having a spare card(s) handy is like having a spare battery, you're sorted in the event that if you lose data or have any kind of error on one card it won't stop you from shooting. Multiple cards obviously means keeping track of which one(s) you've used but that's the same as swapping out batteries - have a system.
UPDATE (YOUR CAMERA): Manufacturers release firmware updates to keep your system up-to-date with any fixes for errors or problems that are identified with a camera. I'd say it's a good idea to keep it as up-to-date as possible.
UPDATE (YOUR CARDS): Media storage has a limited lifespan and will inevitably need to be replaced from time to time. This is me trying to help you maximise that life expectancy and remind you to be prepared for the fact that they are not designed to live forever. Sorry not sorry.
COMMON SENSE MAINTENANCE: It should go without saying (it is written in the documentation that is bundled with your media storage after all) that you should keep your cards dry and clean, protect them from extreme temperatures, try not drop, bend or puncture them and keep them away from magnetic currents. Storing cards that are not in use in that plastic casing usually supplied with the card should give you a little peace of mind.
PROTIP: I'm sure you've heard a story about someone picking a camera up in the club/on the beach/in the street and not having any idea of hwo to get in touch with the owner (despite the viral tendency of social media these days). A tip: take a picture of your contact details whether it be a business card or luggage tag. Lock it in-camera so that it’s not deleted and hopefully if your camera or card is lost the person who picks it up will get in touch.
Anything you want to ask?