How long is the flight? Are you changing planes? How long do you need to travel altogether?
If this is for a flight under 2 hours you have almost no worries.
The three biggest things to do:
Bring a bottle. Sucking will help clear the ears during takeoff and landing.
Bring extra diapers and food for the little one.
Bring entertainment for the little one.
Bring LOTS of food. Do not rely on airline food for the little one. And extra diapers. More. You don’t know when or where you will get stuck and not having those will be painful. Extra set of clothes incase they get sick.
Gate check a stroller. If you have to change planes, having a stroller to get the little one through the airport is very, very handy. It also helps with schlepping baggage around an airport.
At 9 months the little one isn’t likely to be walking, but if it looks like they will be, I’d recommend the aisle so they (and you) can walk back to the bathrooms a few times with them. Don’t do this a lot and don’t interfere with the service. They will likely be crawling and cruising, so taking them to the back of the plane near the bathrooms is helpful.
If both you and your wife are traveling, book the window and the aisle. Most times the seat between you will be vacant and you can have just a bit more room.
Are you flying at night? If so, get them in their pajamas or sleepwear before you get on the plane.
9 months is tough to entertain, but do what you can. If there are toys they particularly enjoy, be sure to bring them. I’ve found books to be great. They travel well and kids usually will sit quietly listening to them. 9 months may be a touch young for that, though. Bring something new. Don’t forget the return trip. New toys tend to hold attention longer than familiar ones. Wrap them because opening takes time and can be a fun thing to do. It also gives them paper to play with.
Basically I pack my carry on bags full of stuff for the little ones. All my stuff goes into the checked baggage, save for a change of underwear, a toothbrush and maybe some deodorant.
My experience with traveling with tired kids is that you get cranky, tired kids on the plane. I’ve tried to keep them to their normal sleep schedule as much as possible. This is, of course, just my experience. Others will have different ones. You know your kid best.
If I think of anything else I’ll let you know.