Adding texture to your drywall can help hide flaws left by sloppy drywall application or years of damage from nail holes or accidents. Even on smooth walls, drywall texture can add a subtle level of visual interest to your home, and in some cases help with the acoustics, as there will not be as many flat surfaces to bounce sound around. The best texture is added along with the paint to minimize the appearance of flaws.
Several types of paint additives are available from paint manufacturers and home improvement stores. Many of these involve granules of various sizes that you mix into the paint for a sand-like or rougher texture. Always mix paint additives in thoroughly, and apply the textured paint to your drywall with a good quality roller using even strokes in a variety of directions. Otherwise, your wall texture will be lumpy and uneven. You can add your own mixes to paint with varying success, depending on the quality of ingredients. For example, fine dry sand makes a good paint additive, but playground sand does not. You may also need to mix white glue or some other adhesive in with your paint to make the additive adhere to the wall better, because too many foreign particles can make your paint fail to adhere properly. Check with local building codes before adding your own paint texture, because some additives might not meet building codes in all areas.
Get a subtle, soft texture on your walls by adding a layer of tissue paper on top of your drywall. Coat the wall in heavy-duty wallpaper paste, then lay crinkled and straightened tissue paper against the wall and smooth it on with a thinner paste. The most important part of the process is tearing off the sharp edges of the tissue paper to give it irregular, soft edges. You can add even more texture by placing small stencil cutouts into the paste before laying down the tissue paper, so once it is smoothed on, the shape shows through with softened edges. In most cases, the flame-resistant properties of the wallpaper paste saturate the tissue paper, so it is not considered a fire hazard, but because tissue paper itself is flammable, check with your local building codes or your local fire marshal to make sure it is acceptable in your area.
Plaster of Paris makes an excellent thick texture that can be applied to walls before painting or mixed in with a paint basecoat. Apply the plaster with a putty knife, trowel, or similar object, using sweeping arcs in various directions to create a random pattern. Use non-shrinking premixed plaster or spackle for best results. Unfortunately, straight plaster tends to crumble when mixed improperly, so for best results, mix the plaster with paint and white glue to help it stick and give it a base color in case your top layer of paint receives damage. The under layer showing through will be lighter than your top coat because the plaster lightens the paint, but it will not be as stark a contrast as plain white plaster. Note that you might need up to five times more paint than the can's stated coverage when using this method, as the thickness reduces the coverage area. Always test a small mix of plaster, glue and paint first in case the paint has any additives that react badly with the plaster or glue. If you get a strong chemical smell or the texture turns rubbery, find a different type of paint or use a different method.