Is your current project process still resulting in scope creep and missed deadlines? You probably need to use a simplified change management process known to ensure better control over what changes are being allowed into your project plans.
A simple process starts out with having a basic change request form. You should insist upon all alterations or changes being filled out in a request form. Your customers will have a basic version (usually online) while internal employees will have to fill out a more complex version (which you can base upon a software project proposal template structure). Once submitted, all project changes first go to the project manager. They do an initial analysis of the categorization for the change including its priority, severity and the amount of man days involved in implementing (estimated at this point).
Some of the minor tasks which only involve small alterations to the product may actually be approved straightaway by the project manager. The majority of changes will however have to continue onto the next stage of the change management process.
The second element of a simple change management process is insisting on having a weekly steering meeting. Project managers use this to present the latest project changes and discuss which ones are to be included, deferred or rejected from your project plans. As each new change is added to the project plan the project manager needs to assess what impact this will have on deadlines. They should insist on other items in the plan being dropped so that deadlines will not be pushed out or missed. Any major change requests that require financial approval should go to your project sponsor after the meeting for financial sign-off.
With the meeting over, the project manager is responsible for sending out meeting minutes and updating their project plans. Their team members should also be stepped through what the alterations are to the plan and how this impacts the work they were assigned.