Are you shooting from a tripod? For portraits it can be helpful to slow down, and do it right. (Of course, that’s helpful for just about any genre of photography, actually!) 🙂
Also, watch where your focus points are, you usually want the eye to be in focus, so it makes sense to put the main focus point right over the eye of your subject.
If you’re ready to buy a 50mm f/1.8, I would buy it from Adorama, since they have better refurbished/used prices right now. You can buy one refurbished for $190, or Used in Excellent condition for only $170!