Oh, those sensitive little ears! An infant is unable to "pop" his ears to equilize in an airplane. Small children can learn, or can have a piece of gum to help them open those passages.
However, the itsy bitsy ones will need this trick:
Have a bottle or feed ready to go when the plane begins to taxi. You want your baby to be sucking when the plane takes off - that will help her ears adjust to the altitude; a pacifier will also help if you use one.
Make sure you do not feed the baby too early. Be sure you are really about to take off. It can be unnerving to have baby finish the bottle before you actually lift off.