Other than the scrollable parallax jlnr already mentioned, I'd say make the ships a bit more plastic. Now going 3D for this might be overshooting it, but you might be able to get away pretty beautifully with a 3-frame animation for leaning in a certain direction, or something along the lines of the space graphics in this article
, which is also just a fraction of the work a real 3D engine would take. Either way will inject life into your game, and while not necessarily making it more juicy, it will make it seem 'more flawless' - yay for term invention :D
Then, the trail - as impressive as it is - seems off. Maybe it's too long, or the gradient curve isn't right, but it just seems like it'd need to be less apparent than it currently is... well, to me - anyone else? ^^"
Also, from a graphic designer's point of view, I have to add that personally, the saturation seems way too high. Maybe it's personal preferrence only, but if I play a space game, I want everything but 'suns' to be cold, color-wise distant and pale. That would also help with some highlights such as lighting on the ship that could be more saturated in contrast.
Either way, use your own judgement by all means - just giving input :)