The Commander always goes to battle piloting an XV-8 Crisis battlesuit.  There are two levels of Commander to choose from: the Shas'o, who represent the most experienced leaders of Tau hunter cadres, and Shas'el, or their less-experienced seconds.

The Shas'o is the obvious choice; for a scant 25 points more than the Shas'el you get extra points in WS, BS, W, A, and Ld.  The extra Wound and BS of 5 are enough to convince me to pay the extra 25 points; the other stat increases are just gravy.  I would only consider the inferior Shas'el in games of 750 points or less; the Shas'o is just too good.

When it comes to outfitting your Commander, it is important to note that with a BS of 5 it is a waste to twin-link weapons.  Weapon fits for XV-8 suits are discussed in the Crisis entry under Elites, so I won't repeat them here.

When considering Wargear, however, note that when a Tau Independent Character selects a Drone Controller, he ceases to be an independent character and instead becomes a unit.  This is a bad thing.  If your Commander remains an Independent Character, then he is virtually invulnerable as long as he remains within 6" of another unit, but does not join them.  He cannot be picked out as a shooting target, and it is easier to be an invalid target than it is to be tough!  Only choose Drones if you also choose to take a Bodyguard.

My favorite Commander is kitted out as follows: Shas'o, Plasma Rifle, Missile Pod, Shield Generator, Hard-wired Multi-tracker (135 points).  If there are a few points left over, or you know you will be fighting at night, I add a hard-wired Blacksun Filter (138 points), and sometimes I consider a Plasma Rifle/Fusion Blaster combination, especially if I know I will be facing heavily armored targets such as Marines, Sisters, or Necrons (133 points).

The Bodyguard consists of 1-2 Shas'vre, also piloting XV-8 Crisis battlesuits.  These Crisis suits don't shoot any straighter than a regular Elite-slot suit, but they are a bit nastier in close combat with extra points in WS and I.  Of course, close combat is the last place a Tau unit wants to be, but these suits have the advantage of both being able to take hard-wired equipment.

The Bodyguard are a pretty large point-sink, o should only be considered in large games of 2000 points or more.  Another way to look at them, however, is a way to get more Crisis suits without using up an Elite slot -- especially important when considering army selection points for Rogue Trader or Grand Tournaments.  They must be attached to your Commander, but do not take up an extra HQ slot, but they turn your formerly Independent Character Commander into a valid target as a unit.

Possible configurations for bodyguards include: Twin-linked weapon systems, a second weapon, and hard-wired multi trackers, for maximized firepower and accuracy; Twin-linked weapon systems and a shield generator, with a hard-wired Drone Controller and Drones for accuracy and survivability; Two weapon systems, a multi tracker, and hard-wired Drone Controllers for bulk of firepower and Drones as ablative armor.

If taking a Bodyguard, then add Drones to the Commander as well.  A Bodyguard and Commander that all take Drones will have 6 extra Wounds the enemy will need to get through. 

Aun translates as "Ethereal" or "Celestial."  They are the royalty and clergy of the Tau, and are held in the utmost esteem.  Any Tau would gladly give his very life to save an Aun from a flesh wound.

Ethereals are arguably the most important unit in a Tau army.  Many players eschew Ethereals, looking at their admittedly mediocre stat line; but the value of the Ethereal is not measured by combat prowess, but rather by his special abilities.

The ability to re-roll all Morale tests on the entire battlefield is not to be underestimated.  It is of special importance to note that this includes Morale checks you have passed as well as failed; especially useful to get your Shas'la out of a losing close combat.  If they pass the test, use the Aun's ability to re-roll and fail the test so you can fall back when you need to.

The Ethereal's secondary ability is to make any Tau unit they join fearless, passing all Morale and Pinning tests.  It is not always a good idea to attach an Ethereal to a unit, however, as that unit may become a target.  Due to the Price of Failure (if the Ethereal dies, all Tau units must pass a Morale check or fall back), you don't want your Aun to get a scratch.  So it is often good to hide your Ethereal behind a large rock or building, in a corner, out of the way, where your opponent will either forget about him entirely, or have to do some seriously long movement to get in a position to threaten him.

To protect my Ethereal, I generally like to give him a hard-wired Drone Controller and two Shield Drones.  Then I attach him to a unit of Broadsides that will also have two to six Drones as well.  With 6 Drones to get through, plus the Broadsides' Toughness 5 and 2+ Save, the Ethereal becomes virtually invulnerable, and the XV-88s become fearless in the bargain.

When making an army of any size, an Aun is always the first thing I choose.  I would never consider going to battle without one.