Take inspiration from natural sources, too. After a while it gets hard to make box-combinations and still be interested.
This was inspired by a mixture of a dolphin, a dragon, and a dropship from that Final Fantasy movie. I've also done Vasudan, etc, ships inspired by things like plants or (cliched for Shivans) insects. sometimes it's very easy to find truly alien looking stuff in nature.
I always
concept stuff; even if you can't draw (like me

), it helps to capture specific parts or 'feelings' you want to convey in a design. sometimes you want to just have a straight blueprint - sometimes you want to snag an 'idea' and confine it to paper.
and don't be afraid to rip stuff off; but IMO it's better to draw inspiration rather than flat out copy. Both
this and
this were mainly inspired by the Viper Mk.VII (with a mix of other inspirations, some more obvious than others), but rather than do a copy I just used the basic 'principles'; long nose, central guns, 'barrel' body, and soforth.
I'd also always think of trying to concept things on a functional level, at least if they're meant to be human. I've made mistakes before with ships that need a little, um, 'mental flexibility' in certain areas of design. from the size of things like fighterbay 'connectors' to the scale of windows (the latter is of course really hard). One thing that I find helps for alien designs, though, is to envisage how the player will see them; for an enemy fighter, for example, consider the pattern of engine flares on the back and the top silhouette, as that's likely how the player will be recognising that ship. Similarly, try and play on emotions for certain designs. Make big ships intricate to enhance scale; a hexagon looks less visually striking than a ship bristiling with control towers, turret platforms, etc. Fast ships look sleek and perhaps have exaggerated engine size. Civvie ships can look old, chunky, slow - and vulnerable.
And, of course, make it look cool
