to improve strength... get the two fingers on a mousemat, or similar slightly squidqy yet rigid surface, and with a flat palm, press the fingers down as hard as possible for 30seconds, then as above, alternatingly drum your fingers on the surface (as if walking the trigger) but whilst keeping your palm as flat as possible; after about 30seonds, repeat the process... a min or two a day will actually do one hell of a lot.
After this you will find finger speed, strength, and for lack of a better word, stamina; will be increased. You will also find that because of this you will inadvertantly use your left hand for more everyday tasks because it's not as feeble and forlorn as normal
Of course, by far the easiest way is to spend a few training sessions just working on left handed drills