Give yourself more time to traverse check-in & security. It's a lot harder and much more stressful to rush with all the baggage a child needs.
When you get on the plane and see where you are seated, ask if it's possible to get the person sitting in your row of three to move (they usually don't mind!) or ask the cabin crew if you can be moved to a row of three seats if one is available - the third seat is great and gives you extra stowage as well as some more privacy.
If you need to, make a list of the things that you need to have in the child's carry-on bag - nothing worse that realising that something you absolutely must have to hand is in the hold bag. Teddy or whatever is an absolute essential so wash & dry it if it's covered in milk/cereal and is a bit whiffy.
If youngster is on solids, bring something for him/her to eat, but remember that all airlines will do is give you a cup of hot water (to mix cereal or milk) and they cannot microwave a cold meal for you - bring a lot of tissues and wipes for spills/sick.
I would always change the child before getting on the plane - my son absolutely hates being changed in the restrictive plane bathroom and screams like he's being killed.
Don't be too fussed about dressing them before you get on an early morning flight - pyjamas are more comfortable and one parent can dress the child in the bathroom at the arrivals hall while the other goes for the bags.
Similarly if its a later in the evening flight, put them into clothes in which they could be put to bed - makes it easier if they have collapsed by the time you arrive home or in your accommodation.
Tell the child (yes, even a nine month old) what is going on and make sure to smile a lot, point at planes and say 'wow' etc. It will help set the tone for the trip and indicate to the child that flying is fun.
Good luck and I hope you really enjoy it!