One sneaky tactic that i employed once was having 3 trukk mobs in their trukks, while having three units of 10 boys in my army as troops choices...
i made a big deal about sneakily telling my mate (not opponent) that i was deploying my mobs away from my trukks, and attempting to lure him to shoot at my trukks (and thus waste 3 units worth of fire in an army that only had about 6-7 units - he prefers large amounts of firepower in large units, rather than large amounts of small units), naturally i got my mate to reveal this to my opponent just to make sure he knew that was my "plan".
Anyway, i distracted him just enough during deployment so that he thought i had deployed the boys and trukks as one (i hadn't, i deployed the boys as my last 3 troops choices, then my 3 trukks as fast attack), so anyway, he falls for it, ignoring my useless unupgraded trukks (that boosted right next to his battleline), in his first shooty phase, so naturally, next turn i unload and assault (he got pissed off, demanded to see my army list, insisted that you can't assault on the same turn you disembark (after showing him the open-topped exception, he caved), anyway, those 27 Orks and 3 nobs managed to tear their way through his entire army on their own.
Still, its a risky strategy that could fail miserably if your opponent is not convinced(i.e. pays attention when you're deploying)/shoots your trukks anyway because they're targets... also, you can't really use it more than once.
Still, it was a laugh.
Other than that, i recommend using a Kult Of Speed, i once tried deploying 3 units of 5 Warbikes, but they got the amphetamine parrote shot out of them very quickly, they did soak up three units worth of fire, but still failled in their role of giving cover to the following orks long enough to make a difference.
I have attempted on numerous occasions to overcome using large mobs of boys (3 units of 30 sluggas, as well as 3 units of 30 Grots), and that still got massacred before i could get within killing distance.
I play against a Tau opponent reguarly, and, with the exception of KoS, Orks are just not cut out for killing them... we die too quickly however, after some reviewing of this board, my next time out i'm going to try something insane, and probably impossible - out shooting a tau army with Orks... i know it sounds dumb, but i'm going to get a large number of shoota boys as well as flash gits, and simply try to blow the hell out of him by having more guns than him (i doubt it'll work, but you gotta experiment).