How to most effectively arange your abilities on your action bars

I should tell you right now that this is a guide for people who are just going to use the 1-10, -, and = buttons as key-bindings, with the rest being placed on extra bars. If you want a guide on using the letters on your keyboard to use your abilities, look elsewhere.

Like many of my advice posts, the topic covered in this post will be a no-brainer to experienced players, but new ones will find it informative, I hope. I often notice that when people try WoW on my computer, they often put their abilities on the action bar in the order they get them in. While this is alright, it is hardly an effective way of doing things. Thus I decided to write up some guidelines on effective placement of abilities.

For starters, you will probably want to have more than twelve abilities available to you at any given time, and you won't want to keep them on a separate toolbar, which will force you to cycle through them. Instead, add some more via the options menu.
Game Menu (escape key) -> Interface -> Action Bars
This moves those extra action bars onto the screen so that you can use them without needing to cycle through them. You will want to put abilities here that you don't use frequently, meaning situational abilities and emergency abilities with long cooldowns, as well as any abilities that are mostly used out of combat. I put these abilities in order form left to right based on how frequently I use them, although I keep the emergency abilities that could save my life close to the left, even if I rarely use them, so that I can have easy access to them.

However, you will want to put the abilities you use frequently on the action bar itself. If you use the WASD keys to move, you can only reasonably expect to be able hit the number keys 1 through 6 with your left hand.* Anything beyond that, you will probably need to click to use. As such, put your six most important abilities here. These include the abilities you use most frequently, as well as any emergency abilities that you need to keep within a keystroke. As a tank, I keep Growl and Feral charge - bear on keys 1 and 2, respectively, so that I can press them at any time when I need to. Note that this only really applies to tanks and healers; if you are a DPS'er or are solo-ing, you probably won't have any emergency abilities that you will need to use with any level of frequency. As a rule of thumb, any abilities with cooldowns longer than thirty seconds, you will probably want to put on the additional action bars. Putting it on the main bar is just a waste of space.

Once you have your emergency keys bound (if you do bind them), you should order your remaining abilities with a combination of priority and frequency of use. As a tank, I usually use Lacerate the most, followed by Mangle (bear), Swipe, and Maul. However, after growl and feral charge, I arrange those abilities on my main action bar like so: Mangle, Lacerate, Maul, and Swipe. I put mangle first because it is my highest threat ability, so I try to use it every time it is off of its cooldown. I then put Lacerate because I use it so frequently. Maul comes next because I need to be able to spam it when I use it, so it's needs to be within my reach, but I don't spam it so frequently that it warrants being before the other two abilities. Swipe comes last because I don't use it too frequently and when I do need to spam it, I can usually see the situation coming and can prepare myself by getting ready to click it. Although I used physical necessities as my priorities on my bear form bar, you can also prioritize your abilities by damage or mana efficiency. For example, on my druid, I have my healing abilities organized so that the most mana efficient ones are furthest on the left.

To give another example, on my Shaman, with whom I mostly solo, I place Stormstrike first, then Lava Lash, then Earth Shock, then Flame Shock. I place stormstike first because it is my highest damage ability and I want to use it whenever it is off of its cooldown. Lava Lash, however, is my lowest damage ability, but I put it second because I frequently use it twice in a fight, sometimes three times. Earth shock and flame shock are both high on my priority list, since they both cause more damage than Lava Lash but less than Storm Strike, but I rarely use either more than once in a battle, so I put them after the other two. I usually start a battle by pulling an enemy with Flame Shock, and I usually use Earth Shock in battle, so I put Earth Shock in an easier-to-reach position than Flame Shock. The rest of my abilities on my action bar are all situational, so I put them after my main abilities.

One last note: It's helpful to keep similar abilities together on your action bars. On my shaman, all of my shocks are together, and ditto for my lightning and healing spells. On my druid's cat form bar, all of my abilities that give combo points are together, as are all of my abilities that use them. On my paladin, I keep all of my buffs and all of my seals grouped together. You get the idea. If you are really familiar with your class's abilities, then you may wish to disregard this advice; it's your choice.

*Some people change their movement keys to be ESDF, which opens up more of the keyboard for your use. You can do this if you wish, but remember to remap the R and F functions.