Have a bag full of tricks. If you're traveling with a child too young to distract himself, include plenty of small toys, games, stickers, magnets, and coloring books in your bag. (Once on board, don't whip them all out at once or let him rummage through -- take them out one at a time and let him play with each until he's really ready to move on to the next distraction. And leave toys that make noise at home. Your fellow travelers will thank you for it.)
Always ask about available seats. If you didn't purchase a seat for your toddler (kids under 2 years old can sit on your lap), expect a full flight and know your chances of getting one are slim. But ask anyway.
Take advantage of early boarding. When they make the call for travelers who need extra time to board, go for it. Use the time to stash bags where you want them before the overhead bins fill up. Get everyone situated and happy.