You might also want to read the thread I have started a few month ago:
Looking for the BASSHowever, Bryon suggestions are very good and clear and pretty much cover the subject. I have but two minor comments, which I hope he does not mind and also agrees with them:
1) When applying "METHOD 1", it is helpful to play a jazz walking bass line (make a loop with the cd player if possible) and look for the position in the room where all notes of the double bass sound equally loud and tight. This is what Bryon means when he writes "listen for the location where the bass is the most *consistent* across low frequencies"
2) When applying "METHOD 2", at step 2) is the crossover frequency of the subwoofer, of course.
Finally, start with only one subwoofer. The second one you can place following the exact procedures outlined above.
Good luck and be patient!
Paul