I always used to go around the wicket to the left handers, and I was always rubbish bowling to them.
BUt now, Ive started bowling over the wicket, as I would to right handers. Bowl a consistant line and length, just on about their offstump, 3/4 up the pitch, or tweak it a bit so the game reads it as a good line and length. Then swing it a bit away from them. Can take a while, bu after about 10 overs of ths, they'll get frustrated and edge, because they havent been scoring runs. Make sure you have a fielder at cover though!