First take off pieces that you are painting; if they can't come out, then it will really be a pain - you'll have to mask off adjacent parts on your car. If your dash or other plastic parts are slightly pliable or soft, then use vinyl paint. Take the pieces and wash them with soap and water. Also, it’s recommended to use dish soap detergent. This stuff is powerful and will strip away wax and grease, and other stuff that may require removing prior to new paint.
Next, gently sand the parts with 600 grit aluminum-oxide sandpaper to roughen the surface up a bit. Be gentle - no need to use lots of force. If the parts have been coated with Armor-All or any products like that, then this step is especially important. Another option, and one that most bodyshops swear by, are using ScotchBrite pads. You'll need the dark-reddish brown ones (inscribe with an "autobody grade"). Use these to scuff up the surfaces real well - you can find these at autoparts/hardware stores in the paint sections or by the sandpapers.
Next, you'll have to clean the parts of fingergrease and dust, etc. Ideally you should go and buy a liquid solvent designed to remove grease and dirt. This is used by bodyshops - you can buy from paint supply places, and better hardware/autoparts stores. However, for smaller parts, isopropyl alcohol should suffice - just wipe the parts down with the alcohol real well and let the parts air dry. This step should take place both before and after sanding.
Spraying paint directly onto plastic is sometimes not good enough - the paint may not "stick on" to the plastic part and as a result may not even set. Plastic, like many other surfaces, require a primering process to ensure your painted plastic parts will stay that way for a long time, and help the paint adhere to the surface. Don’t use a grey, autobody primer; use a plastic primer which usually sprays on clear. In a well ventilated and preferably warm environment
Types of paints
Remember, there are different types of paint. Some are enamel-based, others are laquer-based, others are latex-based. You want to use the same type of paint, coupled with the same type of clearcoat and not mix and match different paint types, or you can get very poor results. Soft or vinyl-like surfaces need a flexible latex paint or dye; automotive paint will not work well on these applications. They tend to chip and flake off. Harder plastics commonly used in car interiors, such as ABS plastic, can be painted with standard auto-based paint and clearcoat.
A paint "system" is always a trouble-free way to go. This is a family of paint products all designed to work with each other. Usually it includes latex or auto base paints, plastic and regular primers, clearcoats etc all together. SEM is a popular example of a paint system. Ask questions. Try to find a autoparts place that has a dedicated paint counter, ask them questions and you will get good advice for your customizing projects.
For all interior painting a FLEXIBLE clearcoat should be used if possible. This has a flex agent in it and will help keep the paint from flaking. This is especially important for hard plastic areas that are subjected to slight bending/pressure (soft plastic/vinyl should never be painted with auto base paint).
In conclusion, if you have used a product such as “Armor-All” on your interior parts, a product called a "fisheye remover" may need to be added to the paint and be necessary to prevent the paint from "fisheye-ing" or "orange peeling". If you’ve ever painted something and had it bubble up, that is what I am talking about. The pores of the paint cannot breathe due to the oily Armor-All product. Addition of fisheye remover can prevent this.
Once dry, position parts for maximum spray. Sometimes you might want to prop them up on top of something to make sure you get good coverage. You should follow instructions on the paint that you're using, but these would be the general instructions on painting plastics (and painting other surfaces): Apply a thin coat on all the parts from a good distance. Don't spray too close to the parts and use short, overlapping strokes from side to side. Over-application will result in runny, uneven coating. The key is to apply lots of light coats, slowly building up the paint, not trying to coat everything in one or 2 coats.
Before you apply the final coat, use a florescent work light, or make sure the parts are under strong light - you'd be surprised upon closer inspection the areas that you may have inadvertently missed, and corners that need more paint, etc. Sometimes, to get a good coat around corners, it may be necessary to turn parts over once dry and do a coat around the edges with the part upside down.
Once parts have dried for at least an hour or two, or overnight, apply clearcoat to non-vinyl painted parts. Clearcoat will help protect the paint from scratches/scuffs, etc. Clearcoat, unless specially designed, isn't usually meant for vinyl painted-parts. However, you could apply some Armor-All to shine up the vinyl parts after they have been dry for a few days.
Also, it may be of benefit to buy a "tacking cloth" - it's a sticky cloth used to wipe down the parts in between coats to remove dust and lint that settles on the paint. Get it from paint shops, in the hardware/autoparts paint section.
Take your time and don't rush and try and paint everything in a day!
These steps have been tried and used with great success by many; they should work well with all applications on plastic. It's important to keep in mind what sort of environment your plastics will be exposed to after painting, and constant maintenance will be required to keep them looking tops.
-Adapted from prophet_ca, http://www.neons.org/