My daughter (who turned 5 in April) has similar tendencies. I will share some of what is working for me - there has been an improvement, although she needs lots of reminders. The problem I was having really centered around her not letting me talk to her sister, and the fact that her sister is not yet talking - this really clarified for me how little opportunity she gets because her older sister so dominates.
I started by finding a quiet moment with her and asked her about how she and her sister (15 months) are different. I told her about all the special times she and I had together before her sister was born. And that I needed to give her sister some of that same special listening time. Then I told her that I needed her to help me with the baby - she needs help learning to talk and share and I think she is having a hard time because my older one likes to talk so much and is so enthusiastic.
Basically, I was up front and honest - I told her she needs to share the talking time more and she needs to be quieter. I told her if I felt she coulnd't pay attention to those "rules" then I would ask her to go be by herself for a while. Then, I do follow-through. If she interrupts, I ask her to remember to calm down her volume or give others an opportunity If she continues to pester/be loud/dominate I do ask her to leave and call her back after a couple minutes. She got upset once or twice but got the idea pretty quickly.
I also used the point that if you do this behavior with your friends (being domineering) they won't want to play with you.
I think if you make it about sharing time and being fair, it is easier for them to understand, compared ot making it all about talking or noise or loudness. It is easier for you to say "Share and let someone else talk" is better than saying "Don't talk." Also, don't just do it in the heat of the moment but at a quiet time explain what you need to him one-on-one.
Can you believe kindergarten came so soon? :)