11 1/2 hr flight with Toddler, HELP!?!

Hi,
As a lot of you know I am finally coming to England this week, It will just be my 22 month old daughter and I. She has been in her TERRIBLE twos since she was one, but for the past couple weeks she has been extremely irritable, mean, loud, and REFUSES to do anything I ask her (before she would sit when I asked, be quiet when I said "shhhh" etc), now everytime we go out now whether to Target, appts, or the library she causes a HUGE scene, btw., I am No Pushover, I am not one of those moms that beg my child quietly to please do this, please do that etc, I am quite firm but Loving, she has been a difficult child ever since she was born, highly sensitive, didn't sleep through the night until she was 15 MONTHS old!?!?  She still wakes up occasionally and hardly sleeps at naptime (no more than an hour) the Drs say she is a high-need child, that doesn't make me feel any better unless theres something that can help.  My long point is I have a LONG flight with her and I am a nervous wreck.
She was difficult in a average way on all of our other international flights before but now she is So Different, i don't know what to expect. 
Does anyone have any advice (besides "Relax").
THANKS Ladies!

have a backpack with things which she doesn't know what's in it. I tend to do that with Jess ...she gets excited and behaves brilliantly on the prospect of finding out what in the surprise rucksack. I usually add:

1. playdough
2. crayons
3. stencil sets
4. raisins/fruit (dried)
5. audio tapes
6. Jigsaw puzzles
7. favorite blanket
8. flash card games

If you're traveling with SO I would split the load if possible so one can have some rest while the other keeps the little one busy. Sometimes little ones can get fed up getting one on one attention from one parent too.

Thanks Alicia,
Yeah, I usually travel with an "emergency" bag filled with toys, fruit snacks (her Fav!), and saving some for later.  I am actually thinking about taking her to the Pediatrician before we go because she gets chronic ear infections and I wonder if that is whats making her act so Hyper and irritable.  Did you have your baby in England or did you bring her with you on the plane?  I'd like to hear any experiences anyone has about travelling with their kids. 
Thanks

I had her in the UK and travel with her ...I tend to travel on my own with her to visit the parents  in the states or for birthday trips and even to festivals so she's a veteran when we travel ...she's now 5. 

I tend to start preparing her that we are going to travel , what we're going to do, how do I want her to behave , and even amend her diet...ie more salads , fruits no sweets days before . It's a good idea to go to the GP as a friend of my used to travel to Australia with her 18mo old and was given medication..she used to do the reward thing and give him stickers for every stage : behave on the tube to the airport , behave while queuing ,etc , etc. It seemed to work and she never needed to give him a sedative.

To each is own really...

Toddlers have to be the hardest people to travel with.  Here's a good thread on the subject.   

http://talk.uk-yankee.com/index.php?topic=11554.0

wrapping the goodies in the backpack adds to the surprise also :-)

Thanks Mindy for the GREAT link, very informative!  Thanks everyone else for the advice!