Hi,
Since you already nurse pretty often, I would recommend taking Fenugreek, it is a herb known for increasing supply in lactating mothers. In far East, nursing mothers take Fenugreek daily with food, just as a part of their diet. You can find it in a form of a tea or as capsules which are more potent. Some say to use Fenugreek in combination with Blessed Thistle, both can be found in health food stores. Follow dose recommendation on the container, usually you need to take each 3 times a day or more. I have heard that if you start to smell like a maple syrup that is the sign you are taking enough of it. Effects are usually noticeable within 24 hours.
Another option would be to pump in between nursings, the best time is in the mornings when your supply is high.