If I have a hill(which I normally do) I park my X bows on the top, with the infantry in front a little lower. They stay there during the entire fight, cept maybe the infantry. I then take my Cav(Swadian Knights and men at arms) and charge the enemy. I can keep them pretty easily doing this, but I actually started doing this to fight the Rhodoks. I could kill an army of 100+ with about 20 men doing this. (Trick being to stay mobile and keep the enemy milling around while your x bows pick them off, the infantry protect the x bowmen from any one that gets through.)
If no hill, but a river, same thing but the infantry in the river and the X bows on top of the bank.
if a flat plain, have the infantry charge with the cav and have the xbows set up on a flank.