Walls will always become grubby, particularly in a home with children. However, even in a child-free home, keeping the walls clean and free from finger marks can be difficult without strictly enforcing a 'no touching the wall' rule! Obviously, this is both impractical and difficult to enforce.
Should you have to clean your wallpaper, the most effective way is to use baking soda. This will not harm the wallpaper, but is usually highly effective on stains. Alternatively, should the baking soda not work, a solution of half rubbing alcohol and half water may work. Alternative solutions include two tablespoons of hydrogen peroxide to a gallon of water, or a cupful of bleach in a gallon of water. Wet the wallpaper with the solution, and rinse thoroughly. Some newer cleaning technology uses peroxide and detergents, or 'Bio-Ox' citrus detergent to penetrate soiled areas. These can then be wiped clean.
Before using any cleaner or solution on a wall, you should always test on a hidden or discreet area of the wallpaper first, to ensure that the solution will cause no damage nor discolouration to the covering.
The above is all very well for a typically grubby wall, however, what about more specialised marks and stains such as wax crayon, or ink?
Crayon - sprinkle baking soda on a damp cloth, and rub at the stain gently. Baking soda should be gentle enough for both wallpapers and paint.
Ink - ink can be removed using either hairspray or lemon juice. Simply spray the area liberally, before wiping clean. You may need to repeat this several times in order to achieve the best results.
Marker - If the marker is alcohol-based, spray the mark with a non-oily hairspray or use rubbing alcohol. For an oil-based marker, it is possible to apply some lighter fluid (care should be taken, lighter fluid is flammable). For a water-based marker, a mix of washing up liquid and colour-safe bleach or prewash spray. Spray lightly to apply, rub gently and then dry.
Wax - to remove wax from a wall, you should first wait until the wax has cooled and dried. Then, remove as much as possible using your fingers, or a credit card to scrape the wax. Next, using an iron on low heat, hold a tissue against the wax and heat it. The wax should melt, and be absorbed by the tissue.
Pencil - In many cases, simply using a normal pencil eraser will be sufficient to remove any marks. Otherwise, the same techniques for cleaning a grubby wall should be used (baking soda or a citrus concentrate).
Smoke Film - It should be possible to remove a smoke film using a solution of ½ cup of borax, 2 tablespoons of soap flakes, 1 tablespoon of ammonia and a gallon of warm water. This can either be used full strength in a spray bottle, or two cupfuls can be added to a bucket of warm water.
Mould - If you need to remove mould from your walls, there are a couple of options you can take. First of all is the creation of a homemade remedy solution of either 1 part rubbing alcohol and 1 part water; 2 tablespoons of hydrogen peroxide per gallon of water, or a cup of bleach in a gallon of water. Wet the surface of the wall covering with one of these, and rinse thoroughly, repeating as required. Alternatively, you can try a more modern method, using a citrus concentrate such as 'Bio-Ox'. Allow this to penetrate for a few minutes, before wiping the area clean.