I'd say the fist step is to start using version control if your not, and if you are using version control research branching and tagging. Using branching you can keep code that isn't production ready out of production till it is ready, and only merge it into trunk when it is.
This is probably one of the single most important things to prevent the issues your mentioning.
Books:
http://www.pragprog.com/titles/svn/pragmatic-version-control-using-subversion
http://www.pragprog.com/titles/tsgit/pragmatic-version-control-using-git
The Joel Test:http://www.joelonsoftware.com/articles/fog0000000043.html
Once you get your version control under wraps, you should be a long ways towards avoiding the problems you mention.