I got asked to post a critique/tips&tricks, and I figured I'd share them.
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Hiya! I saw your comment and figured why not, so here goes.

Line work & Blocking:
Work on your quality of line. Have confidence in your strokes. The biggest disservice you can do to a piece of work is to do 'chicken scratch' lines for a final piece. They're fine to do lightly for the initial blocking, but you should go back over them with a nice confident line. Always, ALWAYS block out your picture before filling it in, even if you are working from a reference picture. Start with a circle for the head, and the cross-piece marking the center of the face and the angle at which the eyes will be. (If the person is looking up, it will be an up-arc, and the inverse is true if they are looking down.)
When contouring, a key way to really make your lines 'pop' is to make the lines thicker at 'joints' and on the highest (or lowest) point of an arc or a curve.

Shading:
Always shade in the direction that follows the flow of the object that you are shading. For instance: If you are shading a sphere, follow the contour of the sphere- shade in arcs. If you are shading a cube, follow the lines. Always shade in the same direction. (Unless you are doing cross-hatching, but I would suggest getting the hang of regular shading first.) As an example: Under David's chin, you should follow the shape of the chin in the shadow down his neck. Do continual blended strokes for one shaded area instead of several singular seemingly random strokes. Keep them neat and soft.

Hair:
Do some studies on hair. It starts at the scalp and flows to either its end or clip/tie etc. Think of where the 'shine' will be. Start your strokes at the base of the hair travelling towards the end point, stopping at the edge of the shine. Repeat, but start at the end and move towards the scalp, again stopping at the shine. Each stroke should be fast and precise.

There are many really great tutorials on Deviantart and I would highly recommend looking at them and practicing.

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Some other 'artist tips' I can give you that may or may not be useful that I wish someone had told me are thusly:
-Don't blame mistakes on being stylistic. You'll only stunt yourself
-Don't even settle for close enough. Always touch up. It's so frustrating doing that last little bit after you're totally sick of looking at the thing but when you look at it again the next day you'll feel so much better about the piece and about yourself.
-Draw stuff that doesn't seem to matter. Hats, chairs, random bits of hair, lips... Just do copious amounts of studies. Keep a separate sketchbook for it. It's tedious and sloggy but it will make your work look so much better and will look AMAZING for a portfolio review.
-Draw everywhere you go. Draw strangers. Draw rocks. Draw cars. Draw bits of things.
-Never give up. Never surrender.

Good luck, keep working hard, and I hope this helps.