Natural stone including marble, travertine, and even granite are extremely porous. This natural porosity can be addressed simply by regularly maintaining and resealing the flooring, countertop or shower surfaces.
What does resealing do? The goal of sealing stone is to prevent liquids from penetrating and soaking into the stone. The recommended method for resealing natural stone is to "impregnate" the material with a high grade sealer, meaning it works from within the stone, not at the surface. The stone finish is unchanged and the seal within the stone is "invisible." Properly sealing requires the time for proper penetration and saturation.
How do I know if my stone needs sealing? Recently I spoke with a customer whose granite counters would darken whenever they put wet dishes on the counter. That is an extreme case of sealer wear. It is highly recommended that resealing is done well before signs of the surface taking on water appear. An ounce of prevention is worth a pound of cure. As a general rule, you'll want to service all your stone between every one to three years.
What about daily cleaning? A residue free neutral stone cleaner is recommended. Using anything that is too soapy on a porous surface, like grout or stone, can act like a dirt magnet. So going residue free is your best option.
What about polishing? The surface of the stone must not be overpolished, and must not be waxed. Overpolish is most often caused by lack of experience or technique. Using waxes can result in unattractive wear patterns and can also act like the dirt magnet discussed above.
What about maintenance cleanings? Your surface may require a semi-annual or annual inspection. Between sealings, a professional cleaning can go a long way to keeping natural stone looking its best. Its more extensive than daily cleaning, and less extensive than service cleaning and resealing. Maintenance cleanings are part of an ongoing care program to keep stone and grout in great condition.
This photo demonstrates a floor which had discoloration and deposits worked into the pores of the stone. The floor was service level deep cleaned and resealed and is ready for a maintenance care program. 
