I think that northern has the right method, or at least that's what works for us. Tantrums are about control (ie what they can and can't do for themselves) and attention. I try to offer choices as much as possible to give them a sense of control: 'would you like the red cup or the blue cup?', 'Are you ready to leave the park now or would you like three more goes on the slide before we leave?', 'would you like to wear your wellies or your shoes today?' This helps with making them feel in control.
If you still get a tantrum, make sure they are safe, try to react as calmly as possible, continue with whatever you are doing and ignore the tantrum. At this age, after a minute or two I use distraction, generally by reading something aloud, or talking or singing quietly to myself, but not competing for volume. I have found they stop screaming so that they can listen to what you are saying :) once they are reasonably calm again, I give a big cuddle, say well done for calming down, and make sure that I carry on with whatever we were doing (if I've said that we are leaving the park then we leave, distracting by talking about looking out for a doggy or something).
Hunger can be a trigger for tantrums so I always have some little boxes of raisins and cracker biscuits in the change bag too.
Not foolproof, but I find it a pretty reliable way of coping with tantrums. And, if you are in public when it happens, ignore anyone who stares! Parents will have been through it too and won't be phased, and people without children will probably find out all about it one day...
There's a good book called Toddler Training, I think by Christopher Green iirc, it's very sensible advice and funny at the same time. HTH :)