all depends. When they do it during diaper changes I will pin them down and say "STOP IT" in a forceful voice. This works a good portion of the time... sometimes not - so sometimes i just end up wrestling them the whole time.
when they throw a tantrum b/c they don't get what they want - i just say "sorry... you can't have XZY because it's bla bla bla... " and try to distract them with soemthing else... or just let them cry... but I do not ever give in and give them what they are crying about... unless of course it's b/c big brother grabbed the toy they were playing with - then i will make him give it back --- all being fair.
i'm a firm believer in not giving in to tantrums. We did not with DS1 and his tantrums ended very quickly - he hasn't thrown one in over a year at least.
Ignore. If he throws a fit because I won't give him something, I just keep doing my thing until he calms down, then explain to him why it's not appropriate to act that way. (Who knows if he understands me, but it's worth a try.)
If he hits, kicks, or bites (me, DH, or the dogs) because he's frustrated, he goes into "time out" - basically I put him in his PNP for a few minutes, until he calms down. Traditional time out wouldn't work right now because I don't think he's capable of understanding that he has to sit, so the PNP contains him. I'm able to walk away until he's done with his tantrum (usually the walking away greatly offends him, so he's willing to calm down just to get me to come back). When he's done, I explain why he got in trouble and have him hug whoever it was that he hurt (yes, even the dogs) and I tell him to say he's sorry (he doesn't talk yet, but I figure it can't hurt).