I can't express how incredibly important it is to never wing a meeting.
Make sure you have all your creative finished and your presentation prepared in time to do a run through with your account team.
I began to learn this lesson when I was at High Wide & Handsome. My CD there was the only CD I had ever encountered that ran through presentations the night before. I didn't really understand because I wasn't at a point in my career where I was presenting a lot.
Once I moved into a Senior role I began to present to clients a lot. I have learned you have to be prepared for every curve ball question or request. The only way to do that is to have all the materials complete and sit down with your account team and do a quick run through of the meeting. Your account team is going to know what the client will most likely ask, this way you aren't thrown off your game in front of the client. This is good for you and the agency.
So just take the time to readjust your schedule to make sure you have time to run through a presentation beforehand. It will really make a difference.