You don't need any other equipment. Take your camera, a lens, and shoot in manual mode (you do know how to shoot in manual mode, right? You know how to work your aperture, shutter, and iso, right?)
Turn your on camera flash off.
Master natural lighting near a window and outdoors, before you invest in anything else. Maternity and newborn shoots can be done very well with simple window light. After you have mastered your natural light, then bring in the softbox - and treat the softbox like you treated the window.
The fact that you are listing your equipment or asking for tips to make your photos look better, leads me to believe that you may not know how to work your camera on manual and know how to manipulate light. Correct me if I am wrong.
Everything on my website, I could shoot with any camera, natural light or with a soft box (although both my natural lighting and softbox work can be interchanged and shot with either), depending on the image. It's all about knowing how to work a camera and whatever lighting is there, not necessarily investing in more equipment or magic buttons.