This is an unusual but interesting question. I assume that you are not interested in mechanical humidification.
Every structure has a certain amount of natural humidification just from occupancy. The average family of four produces 8 - 12 quarts of moisture daily from perspiration, respiration, cooking, cleaning, pets and plants.
- New construction doesn't require humidification since most homes are so tight they develop excessive humidity.
- If your home is excessively leaky it will have a tendency to dry out during periods of low outdoor humidity, requiring added humidity to improve comfort.
To naturally increase indoor humidity levels you should begin by air sealing the house to help keep heat and moisture indoors.
- Once that is complete you can increase the number of plants in the house.
- Plants will increase moisture in the air (naturally) and in some cases improve indoor air quality.
Natural humidification is difficult to control so you'll need to keep an eye out for problems associated with over-humidification. Sweating windows and/or mold formation on walls and window frames are indicators that you have a high humidity issue. You can reduce levels by increasing the ventilation rate in the home by running bathroom and kitchen (if vented to outdoors) exhaust vent fans.
While natural humidification is an admirable goal you might be happier using a room sized mechanical humidification system that gives you control over the amount of moisture put into the air. Humidifiers need to be cleaned and adjusted regularly to prevent mold and bacteria growth in the machine.