You must have a different psychie to me. The couple of preg scares I had had in the past had always been negative and Period came next day. Therefore when I actually was preg I convinced myself it would be neg and that it was a waste of time - I tested when I was a week late!
Just will power I guess. I stopped testing after several months like brettgirl2 described. Although the month that I feel pregnant, I tested early as this is my 2nd and I felt very out of sorts and recognised the symptoms.
The less you stress about it, the more likely it is to happen.