Honestly, the best way I've found to deal with tau is to have adequate Los blocking terrain. Unfortunately, not everyone has a large enough piece of Los blocking terrain. I'm talking high enough to hide a leman Russ behind and long enough to hide 2 chimeras back to back.
No amount of tactic or strategy works against an army that has just about every counter in the book.
Opponent has cover saves?
Use marker lights
Opponent has fliers?
Twin linked and sky fire shots
Opponent has armor/MCs?
Tank hunters / monster hunters
Opponent has hordes?
Large blast high S low AP + ton of high S shots
It's really sad, but true. Want to survive against Tau? Don't let them shoot you and the only way you do that is to have enough Los blocking terrain.
6e is the shooting army's edition and Tau have lots of chances to shoot from interceptor to overwatch, medium to long range shots, while at the same time everyone is scrambling for every bit of cover they can get, Tau just laugh and deny, while claiming cover themselves.