Before you can effectively clean your leather furniture, you have to know whether you have the type of leather that stains can permanently penetrate. (dyed leather) or pigmented leather) This can be ascertained by letting a drop of water fall on an inconspicuous part of the leather. If it soaks in immediately, then so will the stains and if it doesn't, you at least have a chance of removing them. In general, leather should not be too wet and most water-based stains like cola, red wine and mustard can often be removed with a damp cloth. If that doesn't work, try a cloth dampened with a very mild soap and water solution.
Saddle soap is a fine all purpose leather cleaner. Rub the lather into the leather using a cloth dampened with water and once dry, buff with a soft cloth. For effective conditioning, mix 1 cup boiled linseed oil and ½ cup white vinegar. Shake well and apply sparingly with a damp cloth and when dry, buff with a soft cloth. Olive oil can be substituted for the linseed oil. To remove mildew, apply a little antiseptic mouthwash to the area with a soft cloth. Sounds strange but it works.
Don't take a chance with dyed leather and call in a professional. With suede you have to be very careful, sometimes marks can be removed by applying steam from a clothes steamer and a very light rubbing with an emery board. If in doubt, call in a professional.
Sofas should be kept away from heat sources like radiators, and direct sunlight. Heat can cause the leather to crack.