I'm using GypsyMod to give me more buttons in less space. I tend not to think of rows as ways to organize groups of buttons, but left-hand and right-hand, since in fights I tend to use the mouse a lot to start tapping party members and mobs. I put a lot of what I really need on the left, 1-6 and shift-1-5. 1-6 is stuff I fight with; attack, wand, and the mostly-used damage spells. (I've been using Mind Blast less and less, so I should put it on 6 instead of 5, maybe. Then again, it's become habit.) Healing, which I use less often, goes on shift-1-5 in the order that I'll probably use it: refresh, flash heal, then group-heal and then my three most powerful Heal spells.

Right-hand stuff is stuff I don't need to use as often, like buffs, dispells, caster-affecting bits like Silence and Mana Burn, and the more unusual priest spells like Mind Control, Shackle... also Smite since I don't really use it any more, being Shadow-spec'ed, but I don't want to forget it exists, either.

Stuff above the backpacks are useful things that rarely get used: resurrect, human's perception trick, Feedback to spike my weapon... Fade is also there, since mostly I used it as a way to sneak past mobs, but recently I realized its real use is to keep aggro off you if you have to dump a lot of healing fast. That'll probably be moved left somewhere Real Soon Now.

Third row from the bottom is utility stuff. Mount, potions, bandages, Murky, and Abolish Disease because I ran out of room on the heal row.

Top row are crafts and fishing, the panic buttons so I can mouse-click them faster (Desperate Prayer, Levitate if I ever pull a Cathan), my Shadow Form button, and then any wacky stuff like the ball, dynamite, etc. It's also good for temporarily storing buttons if there's an action as part of a quest.

I started putting food and drink on the right-hand side recently, because I kept forgetting I had it. And I have SO MUCH (thanks to my cooking habit and to Cathan) that I need to keep track of it. It's organized by how much health/mana they heal, and if there's a well-fed bonus I stick that above the rest of the food in its heal-range.

Using Gypsy to contract all the buttons gives me room on either side for chat and battle log windows, which keeps them low enough that I can get a really good view of what's around me. (I have trouble reading them sometimes so I keep the background behind them at about 50% or more, but when you do that you can't see through them and can't see as much to the sides. Keeping them low fixes that problem.) Since the chat window starts low, sometimes I'll increase its height so I can keep up with scroll or if I want to get a whole conversation in for screenshotting and posting. :)