I have a pencil cactus that I LOVE b/c it's so hearty (and i have a tendency to kill all things green). However, since I'm not a green thumb I know nothing about "pruning"...literally. I need some guidance on how to prune my pencil cactus so it will grow more from the bottom instead of getting top heavy. Is there a particular part that you cut or does it matter? Do you cut off the new branches or old?