A common mistake I've been seeing in my games against various opponents is the leaving a unit or two units alone and isolated from the main force. Opponents will send a squad way ahead of the main advance, infiltrate them out into nowheresville, deep strike a unit behind your line alone or even leave shooty units behind as the rest of the force moves towards the enemy.
Units left on their own are easy prey for Tau or any army mobile enough to apply killing power to a target unit quickly. Units left alone have no back up. There is no consequence for attacking them as there are no nearby units to counter attack. Time and time again I have been able to rack up kill points by going after isolated units while the rest of the army is too far away to counter attack. Or how about when a squad deep strikes right in front of your gun line without nearby support, easy kills brought conveniently right to your doorstep (rapid fire range).
When a unit is part of a large force, he is protected by his comrades because nearby friendly units discourage your opponent from going all out and opening themselves to counter attack. They are reluctant to commit fully unless they can protect their own units from counter attack.
When setting up your deployment, consider how your force is going to move together and attack all at once as a whole, rather than individuals. If you are thinking about a unit as an individual and what it can do as single unit, that unit is doomed. A classic mistake a regular opponent of my mine makes is. In his deployment, he sets up his Dark Reapers in his deployment zone, in cover or on a hill with a great field of fire. When the game starts, the rest of his army flies off to attack leaving his Dark Reapers eating dust. My kroot then outflank, move on the board, rapid fire the Dark Reapers and follow up with an assault the next turn. If my Kroot don't come on to the right board edge, I'll have a devilfish loaded with fire warriors and some accompanying piranhas go after them. Easy prey.
The lesson is, attack in a coordinated fashion and your army will dominate the board. Strike with as many units as possible all at once rather than piecemeal. The Tau can only prosper if they act as one.