This was recently asked on this forum but I also got an email from a pitcher wondering the same thing:
How can a pitcher work on throwing without a throwing partner?
The more I consider the question, the more I think you really can't. The feedback you get from throwing with a partner is really important and something most of us probably take for granted. And you just don't get that feedback by throwing balls into a net or against a wall or throwing across the outfield to no one but the warning track fence, etc. Isn't this one of the biggest problems with baseball anyways? Unlike basketball, for instance, where you can shoot hoops all day long by yourself, with pitching it's really a partner sport.
What do you think?