If you're installing the grout, the best thing you can do up front is to use a good quality grout sealer to protect the grout.  If your tile has been around for a while, there are some steps you can take to restore the appearance of your grout.

• Prevent stains by taking care of any spills that may stain the grout right away.  Letting them sit will only make them harder to remove at a later date.

• Some homemade solutions may work well to remove stains, grime or mildew. 

•  Mix a solution of warm water with dishwashing detergent or white vinegar to clean.  Prevent and remove mildew with a solution of one part bleach to four parts water or hydrogen peroxide.  Apply the solution to the grout, allow it to sit a few minutes to penetrate the dirt and then scrub gently with a toothbrush or soft bristled brush.  Be sure to rinse the area well with clean water.

• If the stain is persistent, you may need to step up to a commercial grade grout cleanser, like Jasco Grout and Mildew Cleaner.  Scrub gently with a brush, taking care not to loosen any of the grout.  The Clorox Bleach Pen and Oxi Clean powder can both help to brighten dingy grout. 

•  White grout can be renewed with grout whitener by Tile Guard.  Some grout products may not be safe to use on colored grout, so be sure to test an inconspicuous area first to prevent any damage.

To keep your grout in top shape, reapply a good quality grout sealer every six months or when the grout becomes porous and hard to clean.