A saline nasal spray is worth it's weight in gold! Single digit humidity comes as a hell of a shock if you're not used to it. Nosebleeds can be easily avoided with a nosey-huffer of salt water.
Drink water NOW. Start to hydrate yourself at a rate of at least 3 liters a day as soon as you can, well before you set off. It's much easier to stay hydrated than get hydrated.
There's an excellent article on Rokslide right now that goes into detail about this.
As has been mentioned, Diamox makes life so much more bearable above 7k!
Electrolytes are important, so take drink powders in your daypack. I have a 3 liter bladder + a 32oz Nalgene bottle. The bottle is for drink mixes, the bladder purely for water. You seriously cannot drink too much at that altitude.
I've spent a few weeks at over 10k & the first week of every trip is still miserable.
The one thing no-one has mentioned yet is sleep. The first few nights will suck, unless you're well prepared. Again, hydration & Diamox help a lot. You want to be waking up a couple of times to pee. If you're not, you'll be waking up every 2 hours trying to breathe & get back to sleep with a splitting headache.
Salt is also your friend. There's a reason that Mountain House use a ton of sodium
You will lose a tremendous amount through sweating & exhaling & it needs to be replaced. Eating salty snacks (with a LOT of water) will replace this. If you don't, it can lead to muscle cramps & headaches again.
I know I'm making it sound like it all sucks, all the time, but it really is amazing being up that high. You just need to be well prepared.
As far as pre-existing conditions go: I suffer from osteo-arthritis & I've found it actually improves when I've been out West. Whether it's the altitude or the excercise, I don't know but it hurts noticeably less. Allergies are a pig for me though. Different pollens & grasses than you're used to can affect you more than the stuff at home.