It is easy to assume that an account manager's job is to cater to their client's every whim. Whilst "keeping your client happy at all cost" sounds like something out of an A+ customer service manual, saying "yes" to your client's every request may not always be in their best interest (or yours).
You may need to say "no" to a client when their request:
It is important to remember that your client thinks their request is perfectly acceptable. If you are going to "push back", then you'll need to ensure your client feels that they have been listened to and that their request was valid.
It is also important to remember that when you say "no" to a client you are either creating a new problem for them, or pointing out an existing problem that they didn't realised existed. Therefore, a push-back should always be accompanied by both a reason and an alternative.
To make the push-back as positive and constructive as possible, it is important to plan out how you will cushion your response. You could:
Saying "no", or "pushing back" need not be a negative experience. Your client is paying your agency for their expertise, so you can use this as an opportunity to showcase your strategic thinking and how much you care about their business. The more you understand and appreciate your client and their business, the more you will master the skill of saying no like a pro.