As far as MMA training goes, I covered a number of dirty fighting tactics with Perry Kelly Sensei, though he didn't present it as such.
Perry would demonstrate one technique after another. After each I would ask him, "Is that legal in MMA?" and he would answer, "It doesn't matter. Your opponent is out there to hurt you. You should be willing to do whatever it takes to protect yourself." He explained that it doesn't matter if a technique is legal or illegal. What matters is what you can do without getting caught. And even if you do get caught, you generally only get a warning as long as the technique isn't so devastating that the other fighter can't continue on.
This was demonstrated in the fight between Cheick Kongo and Mostapha Al Turk in the recent UFC 92. Al Turk accidentally (or not so accidentally, it can be hard to tell) struck his knee in Kongo's groin. In the same round, Kongo responded in kind and intentionally struck him back in the same way. His response makes sense if you look at it the way Perry does.
"It can be well worth it to knee an opponent in the groin," Perry explained. "All you get is a warning and while your opponent is still recovering from having the fight knocked out of him, you end up winning the round." If this is the case, Kongo was smart to respond in kind so that the two were fighting on even ground, even though it was a dirty and illegal move.
Perry showed me a few other, very subtle things you can do both standing and on the ground that are questionable for rule abiding fighters. One of these includes doing a horse bite (i.e. a pinch with your full hand) to the flesh on the side of the body. The sharp pain that results can serve as a great distraction as you fight. It is against the UFC rules, but as Perry put it, it only matters if the ref sees it, and that type of pinch is hard to see. And even if he does see it, you would only likely get a warning. You're also not supposed to put your fingers in any cuts or lacerations that your opponent may have, but you see people do it "by accident" all the time in the UFC.
It is good to be aware of what types of dirty tactics people might end up using as they fight. And it can be competitive to initiate the use of dirty tactics, but be warned: If you fight dirty, your opponent will likely respond in kind, just like Kongo did.
My personal message to any of my potential MMA opponents: I will not initiate dirty fighting, but know that I am very capable of fighting dirty. My style of Jiu-jitsu the one I have trained in for 15 years specializes in self-defense for the street in which there are no rules, only results. If you take the fight to that level, I won't hold back.