Two golden rules on using rudder to keep straight - especially in the latter stages of the landing roll when rudder authority is low (low airspeed, blanked rudder by fuselage):
1) ANTICIPATE! (Look deep. Use far peripheral vision if no view over the nose).
2) DO NOT OVER CONTROL! (You'll be correcting your own corrections!).
Might sound 'easier said than done', but believe me it will soon be natural!