You can't increase the size of the gaming board. Pretty much all games are played on 4x6 table, so there isn't much room to fall back. Furthermore, the Tau Empire's forces, while highly mobile, are not particularly fast. Therein lies the predicament of every Tau player. How do we keep the enemy at bay and how do we keep him from assaulting our units?
In some ways, I am amazed by GW's foresight in designing the Tau Empire Army. The designers were sure to make 90% of the unit choices, able to move and shoot, and at the same time they gave them a few units that really make the army work as a mobile gunline. I'm talking about Kroot, Gun Drones and Piranhas. I think that while Piranhas may have been an accident, Kroot and vehicle-attached Gun Drones were deliberately included to provide the Tau with a form of mobile defense from assaults.
These three units are important in that they allow you to create distance between your mobile gunline and the enemy. With practice and careful planning, a Tau General can even keep the enemy locked in the mid-range, between 12 and 24 inches of his line, maximizing his firepower while the enemy finds himself fighting through small Gun Drone units, Kroot and Piranhas Light Skimmers. Each of these units is there to block and limit the enemy's movements.
Here's how it works against a mechanized army. Below is a diagram of deployment before the game has started. Let's assume that the enemy has won the roll-off and is going first. The blue represents a Space Marine Mechanized Army. Sorry for the small size here. Click on the image to see a larger picture.
So the enemy has set up, going first, in the middle of his deployment zone because he wants to make sure that where ever you deploy, he's ready to move out to get you. The Tau have set up a firebase in the corner with Kroot on the skirmish lines. The Piranhas are deployed ready to move out to the center.
Turn1- Space Marines Advance, the Piranhas move out to block.
In diagram below, we can see the Space Marine Army has moved up, and the the Piranhas have turbo-boosted out to block the enemy vehicles from advancing further. The drones have dropped and move out to create a layer of defense in front of the Kroot skirmish line.
The Firebases targets the closest enemy transports and with any luck destroys two of them, however it doesn't really matter as you've blocked his avenue of approach. When infantry pour out of the wrecked transports, the drones are there to move up and block their advance as in this diagram below. Once the enemy fights through your gun drones, you move your Kroot up, rapid fire or run to get in position, and make him now fight your Kroot while your battlesuits get to shoot every turn, completely safe from the assault.
This is the basic layout of how to slow the enemy advance allowing your firebase to keep shooting every turn without fail. You use your movement in all three phases, moving in the movement phase, running in the shooting phase and assault moves in the assault phase, to deny the enemy avenues of approach. Successful execution of these "roadblocks" even allows you to whiff with your shooting and have not matter. We all have bad dice some days, but moving and blocking like this, will prevent it from ruining the game for you. It protects your army and allows you to dictate who the enemy assaults and when.
Every Tau Commander, whether playing a mobile list or a gunline list, needs to master these skills in order to for the greater good to prevail. I hope this helps. I'd like to refine this article some more but it's late. Feedback is welcome and appreciated. Any suggestions on how to improve this help as well.