Second of all, sell your food according to your banker/duke/whoever organizes such in your realm, for it benefits you and your knights.
While this is generally a good rule of thumb if you have few allies and no personal political agenda, if you do have allies and do have a political agenda, it might not be advisable. For tips on what to do then, go here:
http://forum.battlemaster.org/index.php/topic,530.0.html
