Hey man - if you want to practice ive got a bit of a work flow example to show you that might help you out a bit.
I see you started with a sketch which is good. It appears however that this very early stage is where you really need to concentrate on practicing. basically youre not going to get any portrait right by just luck if your sketch isnt accurate. I should know i suffer from this pitfall ALL the freaking time.
Im not a master of anatomy but i did do a sketch edit of a girl with roughly the same features that you were going for. I came up with something like this.
Again if youre trying to practice this step is actually one that can be the most time consuming.
Now from here, im going to just use some black and white values to show you how quick building up some form can be.
From here Im just going to use a multiply layer and some hard/soft brush work. nothing really detailed, just try to keep it loose.
now some dimension the other way with some whites on a separate layer.
From here you can build up your image with more detail using similar practices and it makes production not only faster, but more loose as well, as youre not having to worry about repainting something youve messed up. its very hard to paint things with perfect detail on the first pass IMO, so let the build up help you decide shapes and form as they present themselves. Note that shes not really finished or at all perfect but you can get a decent read for where the portrait could develop from here.
If youre wondering, the sketch took me more time than the other steps combined by far.
This should work when using color as well. I know it sounds kind of lame, but if you want to practice, first try practicing with black and white so you can practice your values once youre able to get your form in B&W then you can go charge forth more confidently doing something similar with color. Your art skills will also improve overall id think (i know its helped me a lot).
hope you find this somewhat useful. good luck with your practice.
