First and foremost, I am not a blizzard employee or representative no matter how much i'd like to be, My predictions while accurate every other expansion may or may not be this one, So take some things with a grain of salt.
1) How to make sure your launch day is smooth
Update your drivers, For Nvidia users please see here
For ATI/AMD users please see here
For Intel HD series users please see here
How to check what display driver you need?
If using windows vista or windows 7...
Start>Right click computer, Click properties, Go to device manager, Click display adapters
What if i have an Intel and an AMD or Nvidia? Just update your Nvidia or AMD setup, the driver should have the Intel ones embedded in it (Only needed for certain laptops)
2) Power cycle your modems and other network gear
What is power cycling? Power cycling is where you turn off the device, And wait at minimum 1 minute, And then turn it back on, You will likely need to use a new authenticator code.
3) Reboot your PC
Some of us(Like me) like to leave their PC running, that's all fine and well there is nothing wrong with leaving it running, But to be on the safe side, Do all windows updates and restart your machine.
4) Do a virus and malware scan NOW before launch!
A lot of people don't frequent their account management pages, And with Mop Launch imminent, People may not realize that they have been infected with a keylogger, They go out and buy retail and then get instantly Logged when they try to activate their key with their account, DO A SCAN NOW BE SAFE, Much like it comes to public things, the internet can be a dangerous place without protection, Don't be a jester wrap your Scepter boys and girls.
A list of tools i use personally
Malware bytes anti Malware (http://www.malwarebytes.org/)
Avast! Anti virus (http://www.avast.com/en-au/index)
Both of which are free, DO NOT RUN MULTIPLE ANTI VIRUS PROGRAMS.
5) Acquire appropriate Beverages and snack foods.
It's launch day, I don't normally pig out, But with expansions to WoW i make an exception but as with all things, Take your snack food into moderation, If there is a history of diabetes in the family, Try to avoid things like excess cola and potato chips, As you will feel either sick or dehydrated.
6) It's Just a game, Take a break.
Like you I too am excited to see new content, But remember take all things into moderation, Try to take a five or ten minute break every two hours to stretch your legs, Help blood flow, And reduce eyestrain.
7) General health.
Remember people it is a game, And while some of us are going after hardcore achievements such as realm first, The only advice i can give you is, That A healthy meal that day will keep you going longer then junk food, To reduce chances of headaches or eyestrain, Turn your monitors brightness down a notch, And try to have some ambient light in the room.
8) UPDATE YOUR ADDONS
The #1 cause of problems, Is funky addons, Make sure they are updated, And when in doubt disable them, aswell as Disable addons you don't need, Such as DBM, or Boss timers, Gatherer addons, Mailbox addons.
9) EMPTY YOUR MAILBOX, CLEAN YOUR BAGS
Mail can disappear, I can't vouch for server stability, Expect the unexpected, Not to mention clean bags means less vendor runs.
10) Log out, Outside of a city, It's going to be packed.
No really, Do it.
11) Quest logs, And how to manage!
You'll be out in the world doing 10-15 quests at a time, I and many others recommend Emptying your quest log now of all non essential quests,(Legendary items, And vanity) OR if you want a good head start, Complete a lot of dailies now, Before the expansion launches, Go kill a mob then come back and hand them in.
Non-Essentials gamewise, But totaly important!
1) STOCK YOUR FRIDGE AND PANTRY WITH ALL THE LATEST GOODIES
No really, It's launch day, And it won't be complete without a steady supply of food, There are alot of quick and easy meals to make, And i've got a pot of Chicken soup Just waiting to be cracked open on launch!
2) clean your keyboard and mouse!
This one's fairly self explanatory
3) Clean your work station!
A Clean room and desk means a clean mind and more time and power to focus on the task at hand!
Now there may be a few people that have never played on launch day, OR never played during a release of an expansion (New cataclysm players.)
Firstly, Your latency is going to be higher then usual, Expansion launches are the busiest time for blizzard, They've got the most players playing, the most people watching, The media all over the place, Popular figures playing and all their followers, Expect a higher latency, It is Completely normal, and as with more people, Means more load on your computer, Expect a lower framerate, To increase general performance i would recommend:
Turning shadows off
Turning view distance to medium or lower
Turn Ground clutter(Environment detail) Down
Closing ALL background applications
It is VERY likely that the forums and most account services are going to be DOWN an hour before mop launch to about 10-20 hours after launch, This is due to increased load from millions of people activating their keys, It's completely normal don't panic.
Did you know that you can pay for a faction change or face change NOW and become a panda on launch day? Yes, you can, As the faction change and race change themselves are based on whats available when you press the button in-game, NOT when you pay for it (Check the CS forums for more info)
Good luck, And remember, To have fun.