Irene Gallo, Art Director at Tor/Forge, has posted some guidelines for aspiring illustrators.

Having worked in animation/video games for the past four years, I can second her advice as being sound. I’ve watched demo reels whose packaging was better than the actual content on the reel. The company almost never hires from these. Spend your time on the content of the reel/portfolio, not the presentation of it. If you’re good enough at what you do, the reel could be marked with a Sharpie and could arrive in a beat-up paper bag and you’ll still get the job.

Having said that, spending time on professional presentation should be your second-highest concern, next to working on the quality of the actual artwork.