Actually I wouldn't recomment a window seat. Try to get an aisle seat so your child can get up and down, and not feel trapped. Also, I don't recomment the front-most seats either. Those arm rests often don't fold up and down, so if she needs to nap, she's stuck to one narrow chair, which is not comfortable. Best seats are nearer the middle, less noise than the rear, and more stable (less turbulence) toward the wings. Don't pre-board when the boarding calls come. The less time you have that child imprisoned in a small space the better for her and everyone arund her. Ask an attendant for a couple of extra hands carrying luggage, etc , if you're travelling alone with her, that's what they're there for!! Bring an extra change of clothes, or two (plus sealable bags for dirties), if she's toilet training, or even if she's normally well-trained. Bring zipper-bagged finger snacks, the snacks offered on airlines are pitiful. Remember that little kids NEED to eat every two hours, airlines often don't offer meals or food of any kind more often than every 4 to 6 hours. And wet-wipes, lots and lots of wet-wipes!
One more thing, be willing to bring a little extra cash for if she does happen to have a problem and scream a lot. Buy drinks for the passengers around you in apology!!! LOL Otherwise, Gravol, aka dramamine, is a GOOD thing!!
BTW, I have travelled extensively with 2 toddlers.