Nice way to spark up the Left-Hand renwal topic Juse!
Cherri has a ton of info on this....
Taking from her information start a left-handed note book/journal. Write in it everyday for at least 10 mins. This will build the nerves and the fine control muscles in your hand, forearms and spinal cord etc. The more you write the better your get using/controlling your left hand
The yo-yo or a tennis/raquet/hi-bounce ball will do wonders for your rotation/stroke rhythm muscle and form development.
With CONTROL comes STRENGTH and SPEED which leads to POWER.
Jojo Mayer's DVD and Joe Morello's MASTER STUDIES I & II used on a E-PAD brand practice pad works amazingly well... My testimonial to that fact.
Mark Wessel's Rudimental Techniques section is great for this as well...
http://www.vicfirth.com/education/technique/wessels.html