Use a suede brush to brush off the dirt. you can buy a suede cleaner from a shoe store, or shoe repair shop, follow the instructions. There are products you can buy to waterproof your boots but I doubt you can find anything to protect them from becomming dirty. The sealer would certainly help protect your boots from absorbing wet material such as mud.

Hope this helps: Instructions * STEP 1: Rub suede gently with a bath towel to restore some of its nap. * STEP 2: Remove dry stains and marks from suede by gently rubbing a pencil eraser on the affected area. For wet stains lay a papertowel on top of stain to absorb most of the liquid. Do not apply any kind of stain remover or other products, especially if you want to have it professionally cleaned. This can interfere with the cleaning process. * STEP 3: Brush with a suede brush. * STEP 4: Apply a small amount of white vinegar to a lightly dampened cloth as a last resort for cleaning suede shoes. * STEP 5: Gently rub the surface of the shoe with the cloth, then allow it to air dry. * STEP 6: Remember that stains on leather and suede should be professionally treated. This is always the best option. You never know how the leather was treated during the making of your fabric and what will happen if you try to clean it yourself. Tips & Warnings * Pre-treat suede with a leather protection spray before use. * Leather and suede garments should be professionally cleaned at least every two years. Drop off your garment during the season you are not using it. This will guarantee that your garment is ready when you need it. * Note that suede shoes treated with white vinegar may have a mild odor after cleaning.