When an animal was wearing the skin that now covers your leather sofa or chair, the natural fats and oils in the skin were constantly being replenished. The tanner replaced these oils with others when the skin was tanned, and in the low humidity of your house, these oils gradually leach out. That weakens the leather fibers, and the leather may crack. The most important part of leather reconditioning is replacing these lost oils.
Avoid Vegetable Oils
It may seem like a good idea to replace the natural oils in leather with a household vegetable oil, such as safflower or olive oil, but this is actually a very bad idea. Vegetable oils sink deeply into the pores of the leather, and although they may temporarily restore flexibility, they end up resurfacing and creating oil spots. These spots are unsightly, but worse, they smell like rancid vegetable oil. Instead of vegetable oil, you should stick with products designed specifically for conditioning leather; many of these contain either mink or neatsfoot oil.
Mink or Neatsfoot Oil Conditioner
Leather conditioning oil isn't something you keep around the house for purposes other than leather care, so you'll probably have to purchase some, and there is a bewildering variety of products. Many contain a combination of mink oil and beeswax -- mink oil comes from the fat layer of the small, fur-bearing animals and sinks into the pores to replace the oils that have leached out. You may prefer instead to use a product that contains neatsfoot oil derived from cattle hooves. Avoid liquid conditioners that contain petroleum products or conditioners with other than a neutral pH -- both attack the fibers and deteriorate the leather over time.
You don't want surface dirt and grime to sink into the leather pores when you condition the leather, so clean the leather first with a mild detergent solution, such as one containing an ounce of dish detergent per gallon of water. You can get the leather wet, as long as you apply the conditioner before it dries out. While the water is evaporating, the conditioner will soak into the pores and replace the oils that evaporate along with the water. If the furniture is already clean, you can apply conditioner without washing the leather first.
Apply Conditioner Sparingly
Rub in mink or neatsfoot oil conditioner with a soft, absorbent cloth. Apply a small amount of conditioner to the cloth and rub the leather in a circular motion. Both mink oil and neatsfoot oil should be applied sparingly to avoid overloading the pores that causes the leather to collect dirt. Keep rubbing until the oil soaks in -- don't leave a residue on the surface. Give the conditioner several hours to soak into the leather and, if the leather is wet, give it time to dry out before using it. If the leather stretches while it's wet, it won't regain its original shape.