One really good tip I learned in a Stack and Whack class this last month:
Put a pin in the center of the seam edge you are trying to match. Take that pin and put it in the opposite row center of the seam edge and stand it up (it helps to use a hot pad or pincushion as an extra hand). Take another pin and pin it at an angle across the seam. Take a 3rd pin and make an X with the pin you just inserted at an angle going from the next side back across the seam. You can then remove the pin standing up.
What this does is match the seam exactly with the opposite side.
Its a total pain to do, but the results are astounding. It matches up every seam exactly. All you have to do is ease the remaining seam together and wha-la you have perfectly matched seams every time.
I was taught to do this to match the points of the hexagon star Stack and Whack I was making in the class and it worked! It should work for your postage stamp quilt as well.
Good luck!