Sealing your granite countertops with an impregnating sealer gives protection against stains. A protective layer of a sealant doesn't make granite surfaces completely stain-proof, but it does make it more resistant to stains.
Always use coasters under glasses to prevent any etching. In addition, do not put any acidic foods, like lemon juice or other citruses, directly on the granite counter. Restrain yourself from storing any oil based cosmetics and creams on the stone surface.
In order to prevent any staining, make sure to wipe up spills as soon as it happens. Use a paper towel to absorb liquids that have been spilled. If you want to take it a step further, use a damp cloth with a mix of dish soap to completely get rid of any residue of the spillage.
Granite cleaning contains a lot of acid sensitive minerals. Do not use any products containing lemon, vinegar, or any other acids. You should also stay away from using an abrasive scrubber on any stains. This will lead to etching marks that you will not be able to remove.
