Your demo reel is VERY important if you want to impress casting directors and get work! Casting personnel see hundreds and thousands of demo reels, and you want yours to stand out for being GOOD, not for being yet another poorly put together reel that makes them roll their eyes and wish that actors would actually do their homework and put their reel together correctly.
To start with, here's a Quick Tip video from casting director Erica Arvold, with 3 tips on how to make your demo reel stand out:
To follow up, here's an article by casting director Marci Liroff with 4 tips for the best demo reels:
http://www.backstage.com/advice-for-actors/casting-director-note/4-tips-best-demo-reels/
I have learned some great information from these resources and will be tweaking/updating my own reel soon to make it better. We should always be learning and growing and honing our craft!
