car seat if you can, if she is an infant in lap ticket, ask when you check in to be moved, if there are empty seats, they will accommodate. And even if they tell you at the check in that there aren't, take your car seat to the gate and check again, often times people upgrade to first class and the gate crew has a much better idea of how full the flight really is.
I strongly suggest not pre-boarding, it just tacks on extra time that your child has to sit still. Sigg's bottles are great for flying, they don't leak during pressurization. I take 2 empties, one for water and one for a special juice from the flight attendants, but I water it down and avoid juice and sugary treats, there's nothing worse than a sugar high on a plane. Dress comfy, you are going to be sticky on the other end of the plane and so is your child, so sweats, t-shirts, yoga pants for mom, and Don't forget a change of clothes for your child. I've been doing Boston to LA alone with my son since he was 6 weeks (now almost 3) and it's not easy, but now he's a pro and loves to go on the airplanes.
Some must haves:
extra packet of wipes (skip the hand sanitzer, the bottle just pops open and leaks during pressure changes)
disposable changing pad
a new toy to open on the plane (weebles are fun on the tray tables, but do get dropped alot and little magna doodles are nice because the pen is attached)
blanket and teddy/snuggly
snacks galore (stuff you know she likes, not the time to try something new)
tylenol/motrin
Good Luck!