This tutorial may lack quite a bit of detail, but it illustrates the major points in my approach to portrait drawing.
Drawing from our favorite group of girls doesn't hurt either ^__^ Fans can tell if a Hello! Project member changes her hair color or grows a few centimeters. Knowing what makes them unique shouldn't be too hard, but getting those characteristics on paper is another thing.
Practice techniques by drawing things from real life. Draw your bookbag, draw your dog, draw a tree. Force your siblings to sit for 30 mintues while you draw them. Take what you learn and apply it to something you enjoy. Be it manga, photorealism or caricatures, good drawing techniques carry over from one style to another.