Most airlines, I find, are very accommodating. I travel to las vegas every year with my fiance (who is over 300 lbs) and two friends (one who is just about 400 maybe a little over, and one who clears 500lbs), and if it's not a full flight, the airline attendants may move your seat so that you have an empty one beside you. Of course that only happens if there are available seats, so you can double check with the attendant beforehand at the ticket counter at the terminal (if no one approaches you first). All you have to do is ask the attendant if there are any available seats, and if you might be able to sit next to it to be a bit more comfortable. Unless the attendant is having a bad day and wants to ruin yours, they'll understand why you're asking.
I second the "travel light" tip! Most airlines will allow you one personal bag (purse/shoulder bag), and one medium-sized carry-on (like the rolling suitcases), plus any checked bags. What I do, since I bring a laptop with me, is I put everything I won't need access to while directly on the plane in my laptop bag, and use my personal bag for everything I will need while on the plane. In my personal bag, I make a second bag that contains my e-reader, my ipod and headphones and gum that I can just pull out and shove in the backseat pocket so I can shove my personal bag under the seat. One worry I have is that people will see me as slower than others because of my weight (No one has ever said that to me, but it's just feels like people are looking at me like "hurry up!"), so that's why I like to be able to grab the smaller bag out of my personal bag, sit down and buckle up fairly quickly.
One thing I've learned to carry in my carry-on is Wet Wipes, or any flushable moist towelette, and a travel deodorant. Even though I've flown many times, I still get anxious at the ticket counter and security, cool off at the terminal waiting, anxious again boarding and take-off, etc., and all the sweating and cooling off makes me feel grimy and gross. While on the plane (and even while off the plane...scent lingers longer than we realize) you don't want to use perfume or body spray because in the small enclosure, the smell can get overwhelming to others. But stepping into the bathroom to wipe yourself down, or even just wiping your arms and hands while sitting can make you feel cleaner. Maybe it's just me, I get paranoid because I sweat a lot, and I worry about body odor because of it
Some other things you might want to include are going to be gum/lozenges to help with the air pressure differences, pain relievers (ibuprofen, tylenol, etc), hand sanitizer and tissues.
I enjoy flying, even though it is a tad stressful at points. I'm not a "pretty" flier, I don't look as good when I get off the flight as I did when I got on (my hair, no matter what I do to it, looks like bed-head and my fiance always says I look bedraggled). Wear something comfortable that you can sit in for hours (I love yoga pants for this!), bring a light jacket/sweatshirt because it can get chilly on the flight, and just sit back and enjoy the ride!
Sorry...that got a little long-winded lol!! The only bad experiences I've had with flights are things that are not in my control, like delays and changes. With my heavier friends, the attendants waiting at the front of the plane discreetly handed them a seat belt extender, but at your weight I don't think you'll need it (but if the regular seat belt is a little tight, I'd recommend asking for one...you'll want to wear your seat belt while seated, and the last thing you want is it digging into your stomach on a long flight).
Good luck with your weight loss, I know you can do it!!!! And when the time comes, have a safe, wonderful and fun trip!!!
