Obviously, one should not stop taking asthma medication without talking with your doctor first, since the condition can quickly become life-threatening.
The major anti-asthma herbal remedies tend to be antispasmotics-- they tend to be aimed at helping prevent the spasms of the muscles surrounding the bronchial tubes, and thus make asthma episodes less severe.
The lobelia suggestion would be one antispasmodic agent which might be helpful. Also traditionally people smoked stinging nettles, but that seems suspect to me.
Another one that I sometimes use (also for bronchitis) because of its strong anti-inflammatory activity as well is mullein (Verbasculum thapsis) either as a tea (for daily use) or smoked for an acute episode (however, please don't stop carrying an inhaler without first talking with your doctor).
There is some indication that regular doses of ginko may be helpful and there have been some studies on this, but I have no personal experience with this.
