Expressing gets really hard as your are only making what your baby is requiring. Pretty much the only was to increase it is to pump more often. So as you are already doing, after feeds but also in between feeds as well. It will gradually increase and you will slowly find you start producing more but it can take a couple of weeks of routinely doing it. Also make sure that you pump just as much while she is away from you. During the night your body is super efficient at producing milk due to lovely hormones at play, so if you can try to wake and do a midnight pump, the extra stimulation can help increase supply. You can try natural remedies but it really just comes down to supply and demand, you just have to demand more.