"Plug your armys weaknesses"
Tau has really bad troops, so invest in allies with good troops, like orks or marines.
Got good troops and sucky shooting? Have it the other way around. Tau are actually quite impressive allies, as you get 3 suit squads, 2 of which have str 7 missiles and broadside unit which are actually one of the better AA units in the game right now due to twin linking. Drones make current suits extremely survivable. You can survive bying one FW squad, which aren't super bad due to str 5 shots at 30'', just bad :p
Get IG for bodies, melta and AA (vendetta)
Necrons for extreme flier support
Marines for strong troops and maybe a terminator squad in LR if that is your cup of tea
Eldar for a farseer and a small GJB squad to net you anti psyker and a objective threat
The list of possibilitys go on....
Carry The Emperors Will as your torch!