My 5yr old son and I travel by air 2-3 time a year since he was 6 weeks old, but our flights are usually about 2 hours - though we have suffered a 6 hour delay before! The most recent flight was the first week in October.
The rules are strict now for flying - but if your flight is domestic YOU CAN NOW BRING DRINKS ON BOARD - but only IF they are purchased in the sterile area (AFTER you go through security). The airport stores charge a fortune for things, but you CAN bring snacks from home- just not liquids. Any liquids that you bring through security must be 3oz or less and in a clear plastic bag that you present at the security checkpoint.
To keep your little one distracted, bring NEW toys she has never played with. Other moms suggested the magnadoodle (or however its spelled) and that works great. New books she's never heard before are a good idea too. I used to put plain paper or coloring books in our carryon bag with crayons for my son to scribble with. He also would play with matchbox cars on the little tabletop -- but that has to be up during takeoff and landing. During these times, we'd do fingerplays ("where is thumbkin", "the itsy-bitsy-spider", etc) to distract him. I've never brought a portable dvd player with me, but I'm sure that works wonders. My son, at 5, is a seasoned traveller and ever takes his shoes & coat off as he approaches the security checkpoints and puts them in the bin that goes through the xray scanner!
If your daughter still takes a bottle, give her that during takeoff - it will help lessen the pressure in her ears. You can also use a pacifier, or a sippy cup if it's the kind she really has to suck on to get the liquid out. I normally try to schedule flights during a time my son will be tired, though now that he's in school he doesnt take naps so it's harder. A full belly and the lull of an airplane might help your daughter nap, which will give you a break and make the time pass much more quickly for her :)