Learn to say "no", "not now.." or "Maybe later..."
The first key to establish new boundaries is to accept that you won't please everyone with that. At least not in the beginning. If you have to say "no" to someone, "not now..", or "Maybe later...", it always requires the other to readapt to a new limit.
It's okay. Gaining back control over your life goes with expressing that power in a diplomatic way without truly hurting anyone. You are in charge of your existence and you should really be in control of your time, personal space, activities, belongings, etc.
This gaining back control always means that someone else has to accept that you decide for yourself.
Repeat your message
This is the second key to establish a new boundary: repetition, follow up. Repeating a message is the best way to make sure the other gets it. It works in any situation: profession, relationships, friendships, etc.
Let go of trying to control other people's lives
It will make it much easier to protect your personal space if you focus on yourself. The areas where you can express you "boundary power" are the areas which truly belong to you. Other people's lives, activities, behaviors, agendas is their business. It is their responsibility, not yours.
Enjoy!