Good paint is only as good as the surface to which it is applied. Painting wood is much different from painting metal or plastic. At any rate, good surface prepration is key. Metal and wood should be sanded smooth and ready for primer. Plastic usually doesn't need any primer, but it does need to be washed. Handling plastic coats it with oil from your fingers and hands, and paint will not stick to this.
If any sanding needs to be done, such as for metal, use a very fine 400 grit wet/dry as your final surface preparation. Use a tack cloth or damp rag to remove the tiniest particles from the surface right before you paint.
When purchasing paint, buy the appropriate type for the material you wish to paint. Enamel is a good all around paint variety; Floquil makes very fine paint. However, since it is a petroleum product, check the label and see if it is incompatible with polystyrene plastic. If so, you will either need to purchase acrylic paint, which is water based, or "barrier," to shield the plastic.
To apply the paint, use only high quality brushes. This will help keep fibers and "hairs" from the brush from becoming embedded in your model. If you need to paint a large area, use an airbrush instead of an aerosol if you can. Make sure to follow the manufacturer's instructions for thinning the paint appropriately for your model of airbrush.
If you need to apply two or more colors, modelers find that drafter's tape works better than masking tape. Drafter's tape is manufactured by 3m, and it works better because it doesn't leave the stickyness that masking tape can. If you do find you model is coated with stickyness, I recommend a product called "Goo-Gone." Many modelers also report success with liquid painting masks. With these products, the mask is painted on, allowed to dry, and then painted over. The mask can then be peeled off.
Decals come on sheets. To apply your decal, cut out the decal from the sheet, and then dip it into warm water until it curls. If your decal is small, use tweezers to hold the substrate. Then, place the substrate near the application site. Use either your finger or a blunt object like the eraser on the end of a pencil (so as not to tear the fragile decal) to slip the decal off the substrate and place it on the model.
Flatten the decal using that pencil eraser as best you can. If small bubbles remain, wait until the decal has dried, and then puncture the hole with a pin. A product such as Dio-sol will soften the decal and then shrink the surface to fill the void left by the bubble. Solv-a-set, a similar product, actually melts the decal and fuses it to the plastic. (Tip by Martin)
There is an entire art to applying decals; the basic idea is outlined above. Remember to hide the edges of your decal with a careful coat of weathering above it. In between the weathering and decal, apply a coat of Testor's Dull-cote or Flat to affix the decal and also help hide the edges.