Use White Water and Vinegar

Pour equal parts of distilled white vinegar and water in your espresso machine, and brew as normal but through an empty filter. The emitted mixture may smell bad as it removes all the buildup in the espresso machine. Discard the vinegar mixture after the machine has “brewed” the entire contents.

Rinse With Plain Water

Run two to three more brew cycles of the espresso machine using plain water. The plain water will remove any vinegar residue. Do not run soap or dishwasher detergent through the brew cycle as it may cause corrosion.

Wash the Filter Basket

Wash the filter basket by hand with a mild dish soap. Do not place it in a dishwasher as the metal will corrode. Dry thoroughly with a towel or let it air dry completely.

Source: http://www.answerbag.com/q_view/2126323#ixzz0qwo6HLs1