My boyfriend works part time for a small recording studio that operates out of a house. He started working there close to the time the studio opened and we he started the soundproofing wasn't done yet.
I remember soundproofing the exterior and interior doors was a really big deal because the house was located in a residential area. The last thing they wanted to do was disturb the neighbors! They actually ended up soundproofing the existing door and frame instead of purchasing a new door. It worked really well and was very cost effective.
As the article mentioned egg crate is good for soundproofing- but not just around doors. The studio my boyfriend works for actually put egg crate up on the walls to further soundproof the studio.