You should dust leather furniture once a month.  Use a soft cloth damp with warm water to lightly rub any areas frequently used – pay particular attention to head and arm rests.  You also need to use a leather conditioner on the furniture every two to three months.  You will want to test on an unobtrusive area before applying to the whole piece to make sure it's fully compatible with your leather upholstery.

Leather does acquire a patina as it ages.  This is natural process and not a defect in the workmanship of your leather furniture.  This changing patina of leather is the charm of the material and what makes it such a valuable type of upholstery – it won't stay exactly the same as it ages; it ages with you.  So, even if you clean and condition it regularly, you will eventually notice that the color changes as the piece ages – this is normal!

Removing a Stain
You can usually remove or minimize the stain just by blotting with water.

Often water-based liquids don't really stain the leather – particularly not with the quality of dyes in use today.  Just blot up as much of the liquid as possible – don't rub at it.  Keep blotting until the paper towel or cloth comes back completely dry and then let the leather air dry.

If that doesn't work, try some leather cleaner.Try to avoid place your furniture directly next to a heating source. Avoid placing your furniture directly next to a window, under a direct sunlight exposure. All leathers are subject to fading over time; aniline leathers are especially sensitive to direct sunlight.

Wipe your leather with a soft, dry cloth to remove dust. Fully clean your furniture with a soft cleaner and re-protect with a leather protection cream or spray every four to six months to remove dirt and grime as well as gradual accumulation of body oils. For oil-based stains, please only use a leather de-greaser.

Ballpoint ink and newspapers both have some aniline in the ink formula.  Unfortunately, this means that these inks tend to actually dye leather.  If you find a stain like this, contact a leather professional.