Bokken suburi! Practice basic cuts with the bokken - especially the very basic straight cut between jodan kamae and chudan kamae. It will help you focus on your center - and keep your arms extended.

I know no better way of exercising your centering and your arm extension. Because of the two-handed grip, you automatically tend to move your bokken along your central line. And if you flex too much with your arms, you clearly feel that you lose control of the bokken and the cuts get sloppy.
It is also a good way of exercising center breathing.