Question: Is there a better way to handle unary "-" in converting a infix expression to a postfix one?

The obvious one would be prefix every unary "-" with a 0. Does anyone know better implementation? Thanks!

Answer: 
The way I did this years ago was invent a new operator for my postfix expression. So when I encountered a unary minus in the infix, I'd convert it to `#`. So my postfix for `a + -b` became `ab#+`.

And, of course, my evaluator had to know that `#` only popped one operand.

Kind of depends on how you're using the postfix expression once it's built. If you want to display it then your special `#` operator would probably confuse people. But if you're just using it internally (which I was), then it works great.
Traverse through the string, and replace all unary minus operators with `0-` and surround the result with parenthesis. For example, given `-20 + (-2 * 50)`, transform it to `(0-20) + ((0-2) * 50)`.