Some thoughts...
First, you've done a realy nice job in displaying this bust. Much nicer than a simple plinth.
Add some pupils to the eyes. With a blue as light as that, you want to try for a couple shades, a darker ring around the iris, for example. Also, adding a catch light, a tiny dot of white will give a lot of life to your eyes. Look at the eye on the left of the first picture Gino posted above this to see what I mean.
Next, avoid white for teeth. Go with an ivory or even very light tan with ivory highlights, instead.
Three, work on keeping your colors on what you are painting.. There's a lot of paint where it's not supposed to be, e.g. green on the helmet buckle.
You have some sort of stringy thing across the left *** pocket.
Finally, try to add more highlight and shading across the board. Take a look at the stuff on Planet Figure and TimeLines Forums to see what I mean.