For old japanese swords (or any other swords that employ carbon steel), a light coat of choji oil (or mineral oil) will prevent rust. If the sword is NOT used for practice or cutting, clean the sword once in 6 months.
Cleaning method (usually done on japanese swords) :
1. Using clean cotton cloth, wipe the oil from the blade.
2. Put uchiko powder on the blade.
3. Use a different cloth (same material - clean), wipe the blade from the uchiko powder to ensure the removal of the old oil.
4. Re-coat the blade with choji (or mineral oil).
There are some concerns regarding uchiko powder scratching the blade, well, using higher end uchiko powder will avoid that problem.