Hey games, your penmanship is getting pretty tight. The only thing I can suggest (as I do to everyone) get a few enviros happening . I think for you some organic stuff to balance out the tech would be good.
Like fedezz said, stay away from the airbrush - I use the standard brush (100% hard) with opacity jitter and slightly flattened (so it's not a circle). It really helps to block in values with out getting hung up on rendering things out with smooth gradients. Ones you have all of that down you might use the airbrush in a few places to blend better, but that's more about your personal pref.
Keep it up !