BGP multihoming with two address spaces

I am seeking some feedback/help with my BGP configuration. I am peering with two providers level3 and tw. Unfortunately all of my address spaces are preferring the route over tw rather than level3. I have tried Prepending my AS and the carriers AS to the path on the tw side and I see those update being accepted by internet routers, but everyone is still preferring to install the tw routes rather than level3. I was trying to advertise each provider's address space out their connections and then use the other for backup. Now however everything is coming in through tw and no one seems to like level3. Thanks in advance for any guidance or assistance. Joe