What mirrors have you tried so far?
I was using one of the Blackburn mirrors in some MTB-like handlebars -- it's the third one down, labeled the Multi-Mirror:
http://blackburndesign.com/mirrors.html
That and the Cateye BM-300G (
http://www.cateye.com/en/product_detail/276 ) were the last two bike-mounted mirrors I used before I switched entirely to helmet mirrors.