In general, skin and nail health is enhanced by adequate intake of protein, essential amino acids, and essential fatty acids. Some folks take oral fish oil and unflavored gelatin supplements (or bone stock, as Melissa suggests) for their skin, hair and nails.
You don't say whether the ridges are vertical or horizontal. Vertical ridges are caused by aging, heredity, and sometimes dryness. When you use a moisturizer or coconut oil on your skin and hands, be sure to massage some into the cuticles and base of your nails. Also, wear gloves when you wash dishes or perform cleaning tasks or other jobs with harsh or drying chemicals ( http://www.buzzle.com/articles/vertical-fingernail-ridges.html ).
Horizontal ridges are not always as benign as vertical ridges. They are caused by interruptions in the growth of the nail. This can be caused by physical or psychological stress, trauma, malnutrition, medications (such as chemotherapy), and illnesses (such as diabetes, rheumatoid arthritis or Crohn's disease). If you have recurrent horizontal nail ridges, you should talk to your doctor about it. Ref: http://www.ehow.com/about_5033525_causes-horizontal-ridges-fingernails.html
Other nail changes can sometimes be related to nutritional deficiences or medical conditions. Brittleness can be associated with iron or biotin deficiency, or with thyroid, kidney or circulation problems. Splitting and fraying can be associated with psoriasis, or with deficiencies of vitamin C, folic acid, or protein ( http://en.wikipedia.org/wiki/Nail_disease ).