I have an acrylic positioner from Woodware. It wasn't too expensive and has been invaluable where I want to position a stamp accurately.

I found this clip on You Tube that shows it being used: http://www.youtube.com/watch?v=XxsQkVqa9Fg

There is also a tool called a Stamp-a-ma-jig that does the same thing and loads of demos of that being used on You-Tube.

Stephanie