I used to use the ruler method, and was constantly annoyed at how long it took to get a print centered....then I read Barnbaum's book (Art of Photography) and he has a very simple method for both centering prints, and positioning them vertically on the mat board.
Assuming that you mount similar sized photographs on a standard board size (say, 8x10's on 14x18, or 11x14 on 16x20) you cut a piece of matboard that is as thick as you need it to be to position your print veritically, and the same width as the target mount board. Generally speaking, this strip of mat board won't be more than a few inches 'thick'.
Now, here's the neat part: if you're print is 14 inches wide, and the mat board is 20, that leaves 6 inches of white space. Cut that in half, you get 3 inches - tough math, I know
On the thin strip of matboard you cut, make a tick mark 3" in from both ends. Using a ruler, mark off 5 tick marks 1/16" apart to the left and right of the original tick marks. Number the original tick marks '0', and then the tick marks adjacent to '0' as '1', and so on.
Now, lay the matboard on a table where there's a lip higher than the matboard. Place the 'ruler' at the top so that it's flush left and right. By butting it up against the lip, you'll know it's flush to the top of the target matboard. Then, lay your photograph on the matboard, slide it up against the ruler, and use the tick marks to position left and right (get the edge of the print lined up to the same tick marks). Hold the print with one hand, lift up and edge, and tack it to the mat board.
Done.
It took longer to type that than it will to physically mount the print!
The nice thing is you can make up as many of these rulers as you need from mat board scaps you have left over from trimming boards. I have about 6-7 of these for different size prints and mat boards.
Hope that helps you out!
(fixed a few spelling mistakes)
I think the 'tool' is on the web as a PDF, someone here must have a copy... in the meanwhile I'll try a search
