I was just getting on to say basically what BeMoreFarmGirl wrote. This morning, I asked my (almost-2-year-old) son whether he wanted toast or cereal for breakfast. He said cereal, but then threw a fit when I brought it to him. I tried reasoning with him, which was pointless. Then I brought him a tiny pitcher with a tablespoon of milk in it. I said, "I don't think I put enough milk on your cereal. Can you pour it for me?" And he did. And he was happy, and ate the whole bowl with no more whining. I'm feeling pretty thrilled about that, and I'm trying to think what other things I can let him do that with. Can he spread his own peanut butter? Maybe. PP said something about mess, and she's right. I try to control that by giving my kid only as much as I'm willing to clean up!
A few other things that are working right now (though, I should say, we still have too many tantrums every day...):
Sometimes when he says, "No," I act like it's a joke, like I can't possibly believe he would say "no" to whatever it was. I do it in a silly voice and really exaggerated. I try not to do this too much, because I don't want to discount his feelings, but some of these have now become jokes between us. For example, he loves cereal and would eat it for every meal if I let him. One day, I asked him what he wanted for dinner, and he said, "Cereal." I said, "No," but in a silly voice. He said, "Cereal!" but matched my silly voice. I said, "No," again. Before I knew it, we were both giggling. Now "Cereal" is the setup for a joke, not a tantrum.
The last thing is about him exploring things that he probably shouldn't. I try to think really hard before I take anything away from him. Is he actually going to hurt himself or it? Sometimes, I show him what parts are dangerous (like, on a metal tape measure, the spot where he could pinch his finger), and then let him play. If he pinches his finger, well, he learns. If it's breakable, I ask him to use "gentle hands," and watch him closely. If it's something that I really don't want him to have, I sometimes ask HIM to put it away ("Can you put Daddy's cell phone in his jacket pocket? Thank you.").
When he does something that is really not OK, like banging his silverware on the table (and scratching the table, making lots of noise, and being rude), I take away whatever he is using inappropriately, and say, "We don't [do whatever]." And then I weather the tantrum that follows. Often, he surprises me by only having a very short tantrum and then being fine.
But...yeah. They don't call it the terrible twos for nothing.