Don't get too excited man. Cannabis helps the symptoms of athsma. I can't remember how at the moment, but I believe it's something to do with dilating blood vessels. But while it does make you feel better during the high, the tar you're putting into your lungs can only make the disease worse. It doesn't actually heal anything.

If you really do want it as a daily treatment though, buy a vaporizer. A Volcano vaporizer is the absolutely best one you can get, with the most bang for your buck, and all (or close to) the carcinogens and tar removed. It's very pricy though, about $500-$800. You can get a cheaper one, but they won't be as good (though still MUCH healthier for you than smoking). The amount of crap removed with a cheaper vaporizer can be anywhere from 25%-80% depending on the model. A point of advice in choosing one, there's "convection" vaps and "conduction" vaps. Get a convection vap, they are much better.