I picked up a pair of Pioneer speakers that were in great working condition for $20 at a thrift store. The only problem with them was their dated wood veneer look ... nothing a can of spray paint and a little sandpaper couldn't overcome.
Step 1: Prep
Lay down a drop cloth, unscrew and unplug all the components (taking note of where everything was plugged in).
Depending on the condition of the enclosure, you may want to fill in deep scratches with epoxy and sand down any imperfections. Make sure to wipe off all the dust with a damp paper towel or rag.
Step 2: Paint
After drying the enclosure, apply a base coat (using primer if you have it), allow it to dry (as per manufacturer's instructions on your product), then apply additional coats. Lightly sand between coats, being sure to wipe down any dust or paint residue to prevent cracking or streaks.
Spray painting technique
To keep the coat even, move the can from side-to-side starting well before the piece being painted without slowing down until you have more than cleared the far edge. Keep the speed, distance, and angle constant for the entire stroke. Stop spraying before you begin slowing down your stroke, and similarly, don't begin spraying paint until you're up to speed. Move quickly enough that the layer goes on very lightly; it's better to do lots of light coats (which will dry quickly) than to end up putting on paint too heavily resulting in runs, pools, and drips. Overlap each layer 25-50% with the previous layer.
Step 3: Put It All Back Together
Allow plenty of time for the paint to dry, then put all the components back together. Enjoy your new speakers!