Follow these intructions and you'll end up with a great UI:
Requirements:
A couple of spare hours.
A printer or simple paint program on your computer.
A bunch of addons you want to use.
Instructions:
1) Find a bland part of the game. Simple mountains, sea, field etc. Doesn't matter as long as there is nothing that grabs your attention.
2) Press Alt+Z to get a clear screen and take a screenshot.
3) Either print out a couple of pages of the screenshot or fire up a graphics prog. Whatever youre happier with.
4) Mark out areas on your screen:
a) The "I don't glance much" area, normally the outside area of your screen.
b) The "I watch this all the time" area, normaly a cirlce around the middle of the
screen.
5) Write down a list of your addons. From most essential to least.
6) Draw in the parts of your UI that you don't need to look at much during a figth in the outside area. Normally action bars and mini map.
7) Draw in the bits you want to keep track of. The more important the closer (or in) the "most visable" area.
8) Take several goes until you create an image that includes everything you want in your UI in the areas where you are most comftable with. Ideally, when you think of what you want to look at your eyes go to the spot where you've placed it.
9) Log on and disable all addons. Enable addons 1 by 1 until you've built up your UI to perfection.
10) Sit back and enjoy.
What is this paper you speak of..?