An exterior paint job is only as good as the prep you do first. Make sure to scrape and sand the surface to remove old peeling or flaking paint. Wash off the dirt and dust using a power washer if necessary. Patch small cracks and seams with caulk and apply primer over any bare wood before you paint.
For more on painting, consider:
How To: Paint a House
Painting the House: Should You Hire a Pro?
The Do's and Don'ts of Choosing a New House Color