Are you talking about using css for colors, border, etc. or just trying to get the form elements the same size across browsers?
If you're experiencing size problems (widths) in text boxes, selects, etc. you can minimize the difference by making sure that you're using a monospaced font within the form elements. It's ugly but it does the trick.
As an alternative you could generate the text boxes via javascript and assign different sizes according to the user's browser.
Hope this was helpful.