The only way to enforce any rule is to show a willingness to implement punishment for non-compliance. "if you do this you get a whuppin'". Disciplinary action must be spelled out clearly prior to implementation so that there are no surprises when it happens.
Is this spelt out in the Company CAD manual, and how have you worded it? Any advice would be gratefully received. I have just been promoted to CAD manager 
, but I know implementing these new systems will not sit well with some users who have been here longer than I have
Are you a true manager, with the authority and power of a manager? Or just the guy to blame when it goes mammary vertical? If the former you may have a chance to implement real changes, if the later, run away, far far away.
The existing users are your best allies, viewing them as such, and making them know you do, will smooth a lot of rough water.
BEFORE implementing anything, I would suggest a brainstorming (picking) session. Let 'em know that you are NOT turning this into a committee decision, you're just brainstorming (picking) to see what's going on and to get ideas on implementation. (At the very least it should give you a clue who the problems are.) It will give you an idea on how to implement, when to implement, and which standards are priority.
Let ‘em know that the new standards are NOT suggestions, compliance is mandatory and that failure to comply will result in punitive actions. That being said, standards need to be constructed in such a manner as to make it easier to use the standard than to do it another way;
1.) Drawing setup with a single click,
2.) Layers, text and dims controlled by that same setup
3.) Annotation layers automatically set with every annotation command
4.) Discipline specific layers automatically set for discipline specific functions
5.) Templates with everything necessary already setup.
6.) Etc.
Everything in writing, the standards, how they function, tools available to aid in compliance, punitive actions, etc.
