I sympathize because I have the same problem, and I'm sure it must be almost universal, at least among amateur writers such as we. And if I keep going over a story until I find no more mistakes, I'd probably never get anything submitted.
Goodgulf:
I've tried looking at them in different fonts using different word processes.
I've tried that too. One technique I use is to convert my stories to html format so they look rather similar to reading them in the library. It still doesn't guarantee I will find all the mistakes, but at least it checks whether I've messed up on any inserted italics code. (It's fairly easy to do with a couple of lines of very basic html code.)
The only thing that makes me hesitant to volunteer as a proofreader is that I'm not a grammar expert, and I'd feel bad if I failed to catch an error.