My pal Jeff VanderMeer was tapped to write the new Predator novel and what with one thing and another, he found himself with only two months to produce a final draft. He's written up a detailed set of notes from the process explaining how he did it.
I wrote the first draft of Little Brother, the young adult novel I've got coming out in May, in eight weeks exactly, from the day I got the idea to the day I wrote the last word. I was pulling a minimum of 2,000 words a day and had a couple of 10,000 word days (the book is a little over 100,000 words long). It was weird — the book just wanted to get out — sometimes, it felt like I was passing a bowling ball. It was exhilarating, but I wouldn't want to write them all that way. I finished the book at 5AM in Rome, while on holiday for my anniversary, having snuck out of bed at 4AM to write. Again: wouldn't wanna write them all that way.
Note that some of this advice adds up to "Write some other novels first so you can do your two-month novel in a measured and confident manner."
(1) Make sure your initial synopsis is detailed enough that you can divide it into chapters when you start the actual writing, and, if possible, make sure at that point that you have a one- or two-line description of the action for a particular chapter or scene. Know going into the writing for a week exactly what each scene is supposed to do and why. If you know that, you will find it is still possible to be highly creative and surprise yourself in the individual scenes. If you don't know that, you will spend most of your creative energy just trying to figure out what should be happening. (UPDATE: Jay Lake notes that if he he knew "exactly what what each scene is supposed to do and why" it wouldn't work for him, so your mileage may vary. Perhaps I should clarify in that I just needed to know the action that would occur, more than anything else.)
(2) Make sure you know what kind of novel you're writing. I know this sounds basic, but be able to say to yourself something along the lines of "I'm writing a relatively fast-paced action-adventure story with a subplot involving espionage and a tragic love relationship." More or less a mission statement. You may vary from it, but being able to on the macro level tell yourself what it is you're trying to do is very useful. You'll note my example did not read "I'm writing a multi-generational saga about a powerful crime family." There are some kinds of novels you cannot write in two months.
(3) Make sure you are using a relatively transparent style. I don't believe it's possible to write a good novel in this limited amount of time if you're using a more baroque, layered style (and by that, I mean styles like the ones I used in the stories in City of Saints). This doesn't mean that you can't have complexity of character and complexity of style, but it has to be a more invisible complexity. The layering process, otherwise, will take too much time. In this case, writing a Predator novel, this would've been my approach anyway.