Hands down, the safest way for baby to ride on a plane is in his (FAA-approved) car seat, strapped in to his own airplane seat. By booking your babe his own seat, you won't have to keep a wiggling and jiggling baby still during the flight -- plus, in the event of turbulence, he'll stay put.
Of course, bringing baby's car seat means one extra thing to carry. If you've got a travel system, simply pop your infant seat out of the base and into the stroller. If not, get a bag with handles -- or specially designed straps -- to let you or your partner easily carry the car seat.
If you don't have the extra cash to buy baby his own ticket, ask at check-in if you can sit next to an open seat on the plane, so you'll have a little extra space. Everything booked? Choose what will be most comfortable for you -- the aisle if you think you'll need to walk around and bounce the baby in your arms, or the window if you think you'll need to breastfeed her and you want a little extra privacy.
Finally, if you're bringing baby's car seat but not using it onboard, check it as soon as you get to the airport, so you don't need to lug it around. Good news: Most airlines won't charge you for checking car seats.