Standardize how we make users aware of when they can and can't save

We currently have different ways of telling the user when they can and can't save.  In some screens we show banners at the stop stating what is required.  In other places we disable actions on the page until required fields are entered.  And I am guessing there are other methods of doing this in the application as well.  It would be very helpful to standardize how we communicate to a client when they can't and can't save in most cases.