Some practical hints
Always have a stock of small, surprise toys that you can pull out every time they get awkward. Never give the stuff up front; wait till it is needed. Choose things that are airplane friendly -- that means stuff that is playable on a tray table and doesn't need the entire floor of the plane. Try also to find stuff that is ok to drop on a plane, so that means forget stuff like make-a-necklace bead outfits. Don't rely on electronic things like DVD players because you can't have them on for take off and landing. Toys don't need to be real toys. A pocket compass will keep mine busy for a while.
Beware of stomach issues, so don't rely on stuff like massive hunks of chocolate to keep them quiet. If they spray it all over the plane it is not so pleasant, especially just at the beginning of the take off roll.
Window seats are sometimes popular but don't count on that distraction. In winter low cloud can mean the view is dull. Mine loves window seats but many little ones just don't care.
Seat belts can be an issue. I found one way of preparing for this is to spend some time before the flight showing junior some cockpit takeoff and landing videos on youtube while he is sitting on my lap. As the appropriate moment I pretend my arms are the seatbelt and he has to put it on.
For pre-flight prep I also strongly recommend one of those books that shows what happens at the airport and in the plane. Go a few times through one of these and even the daft security checks with not surprise them (I have seen the security at MAN even look under a baby's beanie cap). Tell them security is a funny game. Which in a way it is.
Finally, never ever use the tray table as a toy to distract the little ones. Once they discover that game they will spend the whole flight making the table go up and down, so the grumpy person in the seat in front will start a new thread here proposing child free flights.