What I tend to do (and this is by no menas to say it's the best way!) is to use Tamiya rubbing compound to even out roughness in the paint. It has to be used carefully, since it really does work through paint very easily, esp on little ridges in the bodywork. Rub clean very well with cotton wool - help for that little extra shine. Then apply decals. Then I use Tamiya polish.
As far as airbrushing is concerned, one golden word. Patience! Also, paint thin layers and thin your paint enough (I think I see some traces of too-thick paint). Wait long enough for layers to dry thoroughly before applying the next. Don't be afraid to experiment. Don't be afraid to strip paint and re-do if you have to. Practise makes perfect: get comfortable with your airbrush and what it will (and won't) do. It need not be on a model: practise on some newspapers or such.
The other thing I see traces of, is some other particles sticking to your paint-job. Try and keep the environment as clean as possible of airborne particles - Newton's 8th law of physics dictate that wet paint on a model attracts all particles it can!
Remember the word patience! Not allways easy, I know...
Keep going, you're heading in the right way.