Chicago to UK is frankly a fairly short flight (~6 hours). My company has a sister R&D Center (Centre?) in the UK so I have done that leg many times, also traveled the same route for vacation.
Beyond that sort of timeframe I start to take other precautions to ensure my flight is comfortable. When I was regularly flying to India & China (up to 17 hours in coach), I would have a couple gin & tonics in the airport, have another with the meal and to wash down an Ambien, then I would be out for 9 hours. Afterwards I'd wake up refreshed with only 8 hours left in the flight.
But if you're not one to sleep, bring plenty of things to entertain yourself with: At least 3 books (in case you finish one and can't get into the backup book, you have a backup backup), a PSP / DS, mp3 player, decent noise-cancelling headphones (have a $30 pair from Target that're 90% as good as the $300 Bose ones). Consider some trashy light reading like a copy of the Enquirer, Mad or Maxim in lieu of War & Peace. Dress comfortably. Take your shoes off & stretch your feet/legs/arms/hands regularly. When they're not serving drinks you can get up and wander about a bit to stay limber. Earplugs, eyeshade, u-shaped neck pillow & your own (thin) blanket are excellent ideas, as is loading up on vitamins before you get on board. But try to travel light - nothing's worse than having to go get your bag out of the overhead again to get something. I wouldn't bother with the laptop. Get on the plane tired if you can.
International coach is a tiny bit more spacious than what you're used to with Southwest. They will actually feed you something resembling food and depending on your carrier you might actually get a few free drinks. If you get to take business class I urge you to do so at any possible cost - you'll actually get treated like a proper human being. They do have plenty of movies and TV shows but they tend to be very 'sanitary' feel-good romantic comedies or sitcom reruns - so don't plan on their entertainment if you find those as odious as I do.
And if you're flying to the UK, try to get a flight on British Airways that's a codeshare with American. BA is head and shoulders above AA when it comes to service, plane cleanliness & making it a decent experience. My preference otherwise is for a Lufthansa-operated United codeshare.
I've traveled internationally quite frequently in the past 8 years - I only have 6 unstamped pages in my passport. If you've got any specific questions, feel free to PM me.