When a new project manager start-off on his project, they have lot of enthusiasm to deliver the project successfully. He wants to make his clients happy , rather delight them, not only that, PM thinks the project will be on-schedule and on -budget. Once the project starts the innocuous looking risks and issues crop up and even though the PM is on top of them , the schedule or cost starts slipping and client starts getting jittery.

Project manager is still convinced to deliver the project and do good job. Some clients try to take advantage of the situation and considering PM is on weak wicket, try to sneak in changes which they did not think about it earlier. In the larger interest of delighting the client the PM agrees for such changes. Whether you follow Change management process or not, practically we always under-estimate the impact of change to other parts of project as every change is one more thing to test and worry about. Sometimes a small change may cause more disruption to project schedule then original scope. It would seem perfectly rational from business point of view to implement the change, but there is always scope to defer it to later phase or release.

Clients may not always be manipulative or take advantage of situation, some clients genuinely ask for changes whether project is slipping or not. If you ask them to categorize as "High","Med", "Low". You will find 80% of requests would be marked "high". An experienced project manager will only ask for priority list of changes. One and only 1 change can be Priority 1, rest has to be marked in ascending order of priority.  The conviction with which you can speak to client comes with certain years of experience. 

An experienced PM knows that changes are bad and will impact the success of project, so they try to push back the changes, unless they find its absolutely critical for business. If there is a workaround, push it to next release. In such cases the project manager knows they can pick only ONE

GOOD CLIENT RELATIONSHIP and KEEPING THEM HAPPY           OR        SUCCESSFUL PROJECT DELIVERY.

As PM you have committed to project's success and not to client relationship. Leave the relationship building to your Sr. Managers or client engagement team. if you try to play dual role then you are headed for trouble.  In short term client may be unhappy and have arguments but when the project is nearing completion within schedule, they will really appreciate your management skills. Your influencing skills will determine how well you are able to SAY NO, while maintaining the relationships.

PROJECT MANAGERS CAN GRADUATE TO PROGRAM MANAGERS ONCE THEY ACQUIRE THIS RARE SKILL.

If you keep accepting changes and feature request, soon you will find that team is slogging and you will be missing the end date. You will then start cutting out  features which will further cause disruptions. Client will be unhappy about the chaos and slipping deadline, your manager will be unhappy that you have over-run the cost., at times if things get out of control the project may even go for termination.  So how do you handle scope changes, feature additions, gold plating etc.

1. Remind the client that project can have limited features in current release and how it will impact the project schedule and success. 
2. Ask client to prioritize the changes from 1 to 100 . Pririoty will decide what changes will be taken up.
3. Ask client to start planning for next release on new Contract. so once this project completes, your team can immdiately work on next one.
4. Understand the fact that you cannot say NO and still keep the person fully happy. 
5. If you have contingency, then towards the end sneak in couple of features to delight the client. They may question, why it was not done earlier, but will be happy anyways. 

LARGER THE PROJECT , MORE THE NEED TO TIGHTLY CONTROL THE SCOPE AND CHANGES. 

A successful release will be biggest reward for you and your team. Confidences go high, motivation increases, client is happy, end users are happy, And all this was possible because you had the expertise and focus required to deliver the project and say NO ( when required).