Virtually anything involving repetition of basketball motions with your left hand is ultimately going to be beneficial to your development. If you're looking for more direction though, I think we could provide that as well.
For starters, and I mean this sincerely, you must forget about how stupid you think you look when you fumble the ball about when you try to finish with your left hand. That is a necessary evil. Many folks do not develop their left hand just because they don't like how stupid they feel when they ultimately make many mistakes while practicing. And to be sure, some people may scoff at your inability, but at some point, instead of feeling insecure about my individualized practice routine, I went the route of, "It's cool, they can laugh now. But they're not working on their game, and that's where I'm going to succeed."
Anyway, I'm betting folks have all sorts of tips and tricks for developing that lefty. Mine are very, very basic. The very first thing I do is I walk through the motions with my left hand. I'll often slowly complete a right hand layup, or prepare to shoot a right handed layup, and I'll concentrate on what body parts move where as well as the finer details such as where my right hand is placed on the ball. I'll take this knowledge and then try to mirror it, simply getting the feel, steps and motions down. Anytime I'm trying to develop a new lefty move, I'll use my right-handed version as a base and walk myself through it.
If you already have all the basics down but are just looking to increase productivity, I say work through left handed layup drills. Work on extending toward the rim. Or try to utilize the Mikan drill: http://www.youtube.com/watch?v=CmFOj0nhUN8
With that drill, finishing while jumping off two feet can be just as effective as finishing off one.
Also, anything involving your left hand is bound to assist in the comfort with which you'll be able to finish. So, left handed dribbling drills, even left-handed form shooting (standing directly in front of the basket, using only the left hand to shoot the ball into the hoop) will help with your left game.