The starting point to preserving a blade is to pick up some light machine oil, the type sold for sewing machines, or some traditional oil for Japanese blades which is called choji oil. This refers to clove oil, but is actually a light machine oil mixed with a tiny bit of clove oil just for the pleasant traditional scent. Do not use pure pharmaceutical clove oil. Many other oils such as gun oils and vegetable oil can gum up, leave stains or have other adverse effects over time.
A few drops on a clean and soft cloth, lightly wiped over the blade when it has been handled should do the trick. In climates that are less humid, cleaning does not have to be done very frequently. Less is more when it comes to Nihonto. Uchiko powder and commercial sword cleaning balls containing powdered abrasive should generally be reserved for blades heavily out of polish, and even then very sparingly. They can be used occasionally on Nihonto to remove old oil, but bear in mind that they are abrasive and will dull a new polish over time. Removal of old oil can easily be done with pure alcohol, otherwise known as dehydrated alcohol or waterless ethanol. This is 99.5% pure alcohol.
Be careful when cleaning the area close to the tang - try to avoid moving the tissue up from the nakago - the tissue might pick up rust particles from the nakago (tang) and scratch the blade. Move the tissue in one direction - from the tang to the tip. Never ever move your hand up and down - you could cut yourself badly some day (and blood will stain a blade.)
Scented or otherwise hampered tissue paper is be avoided. Normal tissue is fine and a lot easier to get a hold of. The best cloth to use is micro fiber cloth, sold for photographic lenses. Microdear is known as the best of these brands and can be found online. These can be washed in water when they get too dirty.
For newly polished blades or newly made blades, the following is the traditional routine:- For the first 3 months, the blade has to be cleaned every week because it can rust easily. After that it is cleaned once a month for 3-6 months and then only every 6 months. It has to be cleaned every time you use it or show it to someone. It has to be cleaned every month for 3-6 months if you move it out of its usual resting place (you move to a new house where the humidity level is different).