I can't believe how many people think that !? is acceptable. These people obviously couldn't write their way out of a paper condom.
First, with regards to the rule that "If the sentence is declarative, use !? ... but if the sentence is interrogative, you use ?!". That's the stupidest thing I've heard since Bush was president. If your sentence isn't interrogative, you shouldn't put any question marks at the end of it? Because that would be obviously stupid? And make everything confusing? The only situation where you might want to do such a thing is when you're trying to communicate that a character has a dumbass California accent?
So rule one is, only ever use this punctuation combination at the end of an interrogative sentence. (Actually that's rule two. Rule one is NEVER USE THIS IN FORMAL WRITING. You might be able to get away with using it in a novel, but even there you should be careful. Also note that if you begin to double or triple it up [?!?!?!], the perceived intelligence of yourself and your writing will drop off faster than EA's employment numbers. None of this means that they can't be used in comic books, of course, where rules of formality largely go out the window, and expressive punctuation works well with the more dynamic visual medium.)
And I'm only talking about putting this punctuation device at the end of a SPOKEN SENTENCE. If you're putting these at the end of a prose sentence, say in a non-fiction essay, you've again jumped the shark of credibility. Your writing will seem ludicrously childish. Use these things ONLY to denote when a character in a story is "yelling a question".
Think about the roles of each of your pieces of punctuation. The question mark exists for a very fundamental mechanical reason: to denote that the sentence is interrogative. The exclamation point, on the other hand, exists only to change the tone in which the sentence is "heard" in the mind: that sentence was spoken in a particularly excited, loud, or adamant tone of voice.
Together their role is clear: to denote that a sentence was interrogative, and that it was yelled. But what is the priority of these two functions? It's obvious: communicating this "emotional tone" of the message is clearly of secondary importance to communicating that the core nature of the sentence is interrogative.
Therefore we can infer that the question mark should ALWAYS precede the exclamation point.
Some people who are even more OCD than myself about English and writing will tell you that this is not a real punctuation device and that there is NO place for it - this is part of why it will kill the perceived formality of your writing. Personally I believe that it fills the same role as the word "y'all". In other words it is perceived as highly informal, but this is an unfair stigma: for it fills a worthwhile, sometimes important, and probably inevitable role in the language.
So in conclusion: use ?!. But use it responsibly. And never, ever, use !?