Cleaning it everyday after using it is the most important step to keeping a flute clean. All you have to do is wipe down the inside and outside with a cloth to remove moisture. Remember that the moisture there is your spit/sweat and it certainly isn't distilled water, so any that dries will leave a residue. The more that dries there the thicker, more noticeable, and harder to clean off it will become. Eventually you'll need to have it disassembled and fully cleaned to remove. It's especially important that the steel hinge rods aren't covered in moisture because otherwise they'll start to rust and bind your keys. If it starts looking black then that's the silver tarnishing and you should only use a dry silver polishing cloth/powder to polish it (after cleaning it of course). If the keys start turning yellow those are spots where plating has come off and revealed the nickel underneath. Often your sweat will do this, adding another residue you want to clean off.
For types of cloth you can use cotton, linen, or polyester. Personally I like to use microfiber on all my instruments as it's streak free. For a deeper clean you can add a little bit of 70%-90% isopropyl alcohol to your cloth. This works especially well for the inner tube and head joint. You can also put some on cotton swabs to clean in between the keys and on the pads, just try not to get too much alcohol onto the pads as it can dry them out. Synthetic oils works better to clean the pads off.
How to remember? Always keep the cloth with the instrument. Always wipe it down when you take a break or finish playing, and definitely swab it out after you're done. Deeper cleaning should only need to be done intermittently.