General Tips
  If possible create your models so that they are in easy to map pieces. For example, placing materials on a car is easier if the body shell, glass, chrome, rubber, metal and plastic parts are all separate objects. This will also make it easier to go back and make changes later.
  Too many high-resolution bitmap textures can use a lot of RAM and slow rendering times greatly. The texture only needs to be as large as it will appear in the final output. Where possible try use procedurals instead or use smaller tiled bitmaps. The Digital Artshop texture collection produces textures in two sizes for precisely for this reason.

Materials
  Materials or shaders can be made up of several different basic properties. The main ones include; Colour, Diffuse, Luminance or glow, Transparency, Reflectivity, Bump, Alpha, Specular, and Displacement. One basic colour or texture could have a bump map applied over it, have transparent areas, given its own glow, be a high gloss or dull matt finish, or even have a decal or logo placed in one specific area. There are many permutations, all applied as layers or properties to the basic material.
  Although basically the same, each 3D program has its own features and way of applying materials to an object which may vary slightly. Therefore the following is just a general guide to the sort of features you likely to find.

Alpha or Opacity
  As in Photoshop etc, the alpha channel is an extra channel. In this case it is used to mask off portions of a texture image allowing whats underneath to show through. The artwork for the mask usually needs to be just a black and white bitmap [JPeg, Tiff, etc].

Bump
  Bump maps are used to control the appearance of small bumps or differences in surface height. They are usually used over the top of a texture map or solid colour. Bump properties are used to simulate the surfaces of things like leather, water, etc.  Applying a bump map does not actually change the geometry of the model.  See also Lighting Effects - Bump Maps
  Below a 'bumpy' bump map applied to a plain orange sphere. The brick wall texture on the two central spheres is more defined with a bump map applied, right. On the far right a treadplate bump map over plain grey.
   

Colour
  The colour settings allow you to set an overall colour to an object. The colour can be picked with either RGB sliders, a colour table or can be set using the RGB editing range (0-255).

Displacement
  This property is similar to a bump map, the difference being that a displacement map actually modifies the geometry of the object.The best results are acheived on a model with a high number of polygons.

Diffusion
  Diffusion restricts the influence of lighting on the surface of an object by subtracting colour. A white Diffusion channel will have no effect where as black will stop light from affecting the surface altogether. One use might be shadows cast over a scene by an unseen object.

Reflection
  This determines the reflectivity of a material and, if nessaccary, the colour of the reflection. Objects with highly reflective materials will act like a mirror, reflecting any surrounding objects.
  Below the bright chrome sphere on the left has both high reflection and specular settings on black base colour. For a polished metal effect, center, both these are turned down a little on a grey coloured sphere. The metalic blue paint sphere on the right has virtually the same reflection and highlight settings as the metal sphere. The blue base colour has a faint amount of noise added to simulate metal flakes in the paint.  

Refraction
  When light passes through a semi-transparent object the rays are distorted. Glass, fluids and other simular materials all refract to some degree. The result is a distorted view of objects behind the refractive surface. Usually a slider control adjusts the amount of distortion to apply.
  Below are three examples of refraction. On the left is a solid glass sphere which has high refraction settings, as it is acting like a lens. Compare that with the lesser distortion through a glass tumbler. Both these also have high gloss [specular] settings and slight reflections. On the right is an ice ball and because water ice is never perfect a noise setting or map is used to slightly distort the refracted image. Also the highlights are slightly muted and a very faint noise map is used for the transparency to give it a cloudy look.  

Procedural Textures
  Procedurals are textures derived from a mathematical formula generated within the 3D program. As well a number of presets supplied with each individual program you can also produce your own. Common examples include wood, marble, checkerboard, etc. Although procedurals can sometimes look artificial they have the advantage generating a random, non-repeating pattern. Another feature is the texture pattern is is applied all the way through the object, so if you were to remove a section from an object the texture detail will still flow correctly.

Left,  A 3D Procedural texture applied to an object which has had a piece removed by boolean subtraction.
Below, this sand procedural texture in perspective shows no signs of  any repetition or patterning at all.

  Below, a typical selection of preset procedural textures. Left to right, top to bottom, wood, marble, elephant skin, fern leaves with transparency, snake skin, multi colour checker, cellular chrome, psychodelic, limestone, rock, rusty metal and wire.

Specular
  The specular properties control how shiney or wet an object appears. It represents the highlight that a light source creates when it is shining on an object. A small, sharp specular highlight makes an object look very shiney or wet, whereas a large, diffused highlight will make an object appear duller and matt.
  A colour can also be applied to the specular properties of an object. Some real life objects have different specular colours to their actual colour. For example brass is a yellowish colour, but has a greenish specular colour.
  The picture below shows a high gloss sphere, left, with sharp white highlight and middle range reflections. In the middle a semi-gloss effect with muted highlight and right a completely matt sphere with just a hint of highlight added.

Texture Map Interpolation
  Texture maps in perspective can sometimes become over-enlarged end up looking pixilated or stair-stepped when rendering. By using special anti-aliasing it is possible to blur textures to hide or reduce this pixelation. Make sure the check box in the materials editor is ticked if this should occur.

Transparency
  This property can make an object transparent. The settings will allow you to adjust the colour and amount of transparency. Usually you can also use a greyscale bitmap to create partial transparency for an object. A good example of this can seen below where the planet clouds, with transparency map are placed on a slightly larger sphere than the planet, giving the clouds the hint of a shadow.

Luminance or Glow
  This is used to make an object have its own internal glow, such as glowing fire embers, a TV screen, or this lava planet, left. Luminance can be appied to the whole object or partially using a texture map. The Luminance properties do not give off light.