It would be hard to tell you everything you need to do for OCR to work, especially 100%. Here are a few things to consider, though:
Make sure your characters can be separated from the background and each other easily using standard thresholding. It is easier if the characters are a solid stroke with medium to thick lines.
Make sure your characters are easily distinguished from each other. The classic case is 0 and O (number and letter), which are very difficult to distinguish unless the zero has a slash through it.
I think for good OCR, the height of each character should be 20 pixels or more. You don't want to get too large, though.
There are a few other things that depend on the application that I can't guess at, such as lighting, camera selection, etc.
Bruce
Bruce Ammons
Ammons Engineering
