practice, practice and more practice. The most important thing is to actually use and train your weaker hand. A lot of streetballers don't even bother to train their weaker hand. once you start to train your left hand it will get better. Of course it's very difficult to get it to a point where your left hand is just as good as your right hand, but minor improvements actually can be achieved pretty quickly.
and as somebody has mentioned before it also helps if you carry a ball around a lot. I think my handles improved the most when I used to dribble in our house all the time. But now I live in an apartment and can't do it anymore