Boundaries are for you, not the other person. You cannot enforce boundaries or force someone else to do something. All you can do is change the way you react. If you tell your mother not to call you crying, you cannot stop her, but you can refuse to take her calls.
Boundaries will not heal the pain of an painful childhood. The purpose of boundaries is to protect you from more damage. If you're trying to use boundaries to eliminate pain from your past, consider therapy instead.
Many people react poorly to new boundaries. Teaching people how to treat you is an ongoing process that must be continually tweaked. Don't be surprised if mom, dad, or your sibling throws a tantrum when you first try to set boundaries. Don't worry; things really can get better with time and perseverance.
Boundaries require psychological strength. It's not easy to ignore someone who is becoming increasingly demanding, so plan on rallying your resources. Remember that therapy can be a valuable resource.
Everyone makes mistakes. If you allow someone to break through a boundary once, this does not mean all is lost. Pick yourself up. Dust yourself off. And try again. You only lose the battle if you give into defeat.