Preparing the grout
Make sure the tile or grout is sealed, if necessary. When tile and grout is sealed with a tile or grout sealer, the grout and tile surface is better protected against water damage and eventual mildew build-up. When unsealed, the grout absorbs the dirt, grease, water, or whatever else comes near it. When sealed, tile grout is much easier to maintain and keep clean.

Certain tiles that are unglazed don't require any type of sealer at all, so it's important to know whether or not your tile even needs it. Check the manufacturer, or ask at a hardware store. In addition, you can test whether your grout needs to be sealed or not by putting a small amount of water on the surface of the grout. If it becomes dark, it absorbs water, making it more prone to mildew, and should be sealed.

Cleaning the grout
When you clean, make sure you wear latex or rubber gloves, as cleaning materials can be harsh on the skin. Make sure you never mix chemicals as well. You will also want to keep the room ventilated, so make sure the door is open or crack a window if there is one in the room. Once the room is ready and you are wearing gloves, take the following steps:

1. Use a commercial grout cleaner or a heavy-duty, high-alkaline cleaner that contains bleach. These are the best for getting grout white again. Try the commercial cleaner first, as the harsher cleansers will be harder on the grout itself. Once you have sprayed on the cleaner, let the grout absorb it for several minutes or however long the package recommends.
2. Use a soft bristled brush or specialized grout brush to clean the area. Harder brushes or wire brushes should be avoided, as they will agitate the grout and can wear it down. Rinse the area when you are done to wash away the cleanser.
3. If you follow these steps and the grout still looks grimy and mildewed, try a deep-cleaning with the high-alkaline cleaner. When you do this, allow the cleaner to soak for a longer period of time.

Maintaining the grout
Once you get your grout back to its sparkling and pristine condition, you can keep it that way through daily maintenance. This can be done by immediately cleaning up spills made on tiles, and by cleaning showers or wiping them down after each use. Several commercial cleaners on the market are designed for spraying on the tile and leaving it there, eliminating the need to rinse and scrub.

Once you have thoroughly cleaned your tiles by following the steps listed above, daily maintenance with a regular commercial cleaner will keep your tile looking clean, new, and mildew-free.