Before I go into detail about how to make a business-site intro movie, I will first explain a little about the basics.
You have to keep the file size of your Flash file as small as possible, so it won't take forever to load (this is discussed in the section "Step 3: Adding a Preloader"). Of course, everybody has his or her own definition of what is "too big," but I would suggest that you always keep the file size of your movie below 400 kilobytes. This can take up to three minutes to load with a slow connectiona long time when you're staring at your screen waiting for something to happen. The things that really increase the file size dramatically are
"Imported" graphics (Jpegs, bitmaps, etc.)
Sound or music
Although these are the two main ingredients for a good Flash intro, try not to use to many of them in one movie.
Keep the color schemes, fonts, etc. the same as on the actual Web site. Don't have a bright yellow intro movie when you mainly use blacks and blues on the site itself.
If your site consists only of Flash, don't have the intro movie "flow into" the main site. Make two movies instead. This is better because visitors are not forced to watch the intro, but can click on to the main site if they want.