If you want a longer-lasting paint job, you'll need to prepare your cabinets first. Kitchen cabinets, especially old ones that have been exposed to airborne grease or smoke from cooking, need to be cleaned well. Even if you plan on sanding the cabinets before painting, they still need to be cleaned. Oil and dirt can be embedded in the wood, and sanding will not remove them sufficiently.
Fill the kitchen sink with warm water and add grease-cutting dish soap. Place a sponge into the soapy water. Dust off the cabinet doors. Wash them down next with the soapy sponge. Make sure to scrub off any greasy residue. Any grease and dirt will interfere with the paint's adherence. Rinse the sponge under warm water and rinse the cabinet doors with the sponge.
Dry the surface of the cabinet doors with a dry towel. Place a drop cloth on a stable surface, such as a table. Unscrew the cabinet doors with a screwdriver. Place the doors on the drop cloth. Check for grime where the hinges were.
Dust all shelves with the dust cloth. Even if you are only painting the outside of the cabinet, you need to do this so the dust doesn't get stirred up later and settle on wet paint. Dust the outer sides and edges of the cabinets. Wash well with a soapy sponge and rinse clean.