1) Use wound allocation to prevent casualties from shooting, use wound allocation to cause casualties in the assault. You want your units to be durable enough to soak up return fire. Luckily, our main firepower units, like Broadsides and Crisis Suits, have a good armor save and access to drones for wound allocation. In the assault however, the last thing you want is to be bogged down. Use wound allocation to suffer casualties so that the assault ends on your opponent's turn, leaving his unit out in the open for a counter-attack in your next shooting phase.
2) Never advance unless you absolutely must to claim an objective. Unless you need to gain ground and claim objectives, there is no reason to advance on the board. It's tempting to do so to get your troops into rapid fire range, but resist, a good Tau player knows when to exercise restraint.
3) Extrend terrain with Kroot Walls, Drones and Pathfinders. See the picture below. I've used my Kroot to extend the cover save provided by the ruins. The Kroot have the majority of their unit in the terrain and therefor the entire unit gets a cover save. Because they are strung out, they confer cover saves to the Crisis Suits, Drones and Broadsides behind them.
5) Fire Warriors ain't for fighting. In my experience, Fire Warriors are most valuable as cheap scoring units in Devilfish. They aren't good shots and so lack the ability to cause significant damage and aren't numerous enough like Kroot to be a durable unit. Keep them in a Devilfish and use them to claim objectives.
6) Devilfish ain't for fighting. Just like Fire Warriors, Devilfish while durable, aren't very capable fighting tanks. While at first you might think that upgrading them with Smart Missile Systems, Targeting Arrays and Multitrackers might be a good idea, look at the points you're investing into the vehicle and do the math. 7 S5 AP5 shots at BS4 from a tank thats costs 120 points is silly. Better to go cheap, get yourself a Disruption Pod and be done with it. After all, the free Gun Drones are more valuable than a 20-point Smart Missile System any day.
7) Build Firebases that have an escape route. When building a firebase, consider your enemies most likely avenue of approach and build in an escape route. You can do this by positioning Devilfish to provide cover for suits as you move, or by using Piranhas to stall an enemy advance while you move-shoot-move your way out of trouble.
8) A good Tau army is like an onion, it has lots of layers and makes people cry. By layers I mean lots of cheap units that can put in your enemies way blocking him from assaults, speed-bumping essentially. My latest Tau build has 19 units in it. About 50% of these are 'layer-units', ones whose survival isn't important and who can move to get in the way of things. Use these layer-units to bait opponents into a kill-zone or simply block him from killing your more valuable suits and troops.
9) Never waste your drones without purpose. Free Gun Drones, or burger-bots, are a Tau player's best friend. But you've got to protect your friends. Like a good friend they'll protect you, too. Always do your best to make them difficult as possible to kill. Make sure they have cover save and only commit them when you need to block an assault from happening. Their little twin-linked carbines will most of the time cause no damage but you can sometimes get lucky and immobilize a Rhino with them. Keep them around, they are an asset that shouldn't be wasted needlessly.
11) You're mobile, not fast. Don't forget that you are playing Tau, not Eldar. Our forces are mobile and can maintain the a high rate of fire on the move, but they aren't fast. Deployment and destroying your opponent are important to ensuring that you can get where you need to before the end of the game. Don't even deploy thinking that you'll have to the time or speed to redeploy if needed. Tau need to clear the board before they begin moving, so concentrate on dealing damage first.
If you've got more Rules of Engagement, please post them in the comments section.