I actually think Microsoft needs to do something radical to help third-parties.
One way would be to reduce their licensing fees significantly (possibly by $5 per DVD). This would give 360 games a $15 premium over Wii games which would help negate lower development costs.
Another would be to provide game engine support to third-party developers directly. This could help reduce costs significantly for third-parties (at least those that chose to make 360/PC exclusives).
And finally they might consider setting aside about $1 - $2 billion dollars to provide low interest loans to companies making games for the 360.