I hate to quote every motocross instructor and training video but ...
Figure Eights,
Figure Eights,
and more Figure Eights.
Make the long and wide, short and tight, long and tight, short and wide, oblong, round...
Practice, practice, practice.
I may be old and slow (vintage bikes against moder machinery - I race a 1975 175 in MX1! All of about 14 Hp) but I am the fastest in the corners. It is really satisfying when a fast kid comes up and says he couldn't pass me in the turns.