L E G O. . . .B U I L D I N G. . . .T I P S
Going overboard with details is especially easy to do with larger models. Just because a ship can hold 112 cannons doesn't mean it should. Too many things sticking out make a model look cluttered and distract from the curves.
Personally, I like to just dive right into the bin, but some people prefer to draw a few preliminary sketches before they start to build, so don't be afraid to try that. Either way, you should have a good general idea of what you want in your head before you build. Don't let the bricks design your model. Make it how your mind sees it and don't settle for what the bricks seem to be limiting you to. Some solutions may take a while to come across, but with Lego, there's always a way. ;)
I forget where I read it, but a fellow AFOL mentioned that models should be done in one primary color, one or two secondary colors, and one hilight color. That pretty much sums it up. Beyond that, they tend to get cluttered.
Also important is not to wander to far in the other direction. I'm a big fan of black when it comes to model design, but too much of any color (especially black, though) can obscure details.
Also, this is where Raytracing can work wonders. The real model of Intruder is actually a pan-spectral mix of mostly white, with red, yellow, trans blue, and just about everything else in the bin, but since I raytraced it, no one will ever know. (haha ...doh!)
Inspiration can come from anywhere. When you walk through the park, keep in the back of your mind the idea that anything can be turned into a space ship (that's the beauty of building in space rather than town or train for instance - reality is not a limit). Scythe was inspired by a meal at an Italian restaraunt, where I saw a scythe sitting along the wall and thought "hey, a ship could look like that."
Don't be afraid to rip yourself off! If you come across something you really like, re-use it! Copying yourself is not cheating. For instance, the cockpit of the Triton was compact, but still had enough room for a pilot and computer/control console. It was also easy to work with, easy to modify, could acommodate a nose weapon as well as a weapon on either side, and IMnsHO, still managed to look rather elegant. In short, I wasn't about to use it in just one model. :) I blatantly ripped it off from myself and built it into the Apollo a year later, and I haven't looked back.
Fear not the technic piece
If you have them, use them! Let's say you want your wings to be at a 20 degree angle, but hinges won't do the trick because they're to heavy and don't stay put. Use technic pieces! At the heart of Apollo's design are technic pieces. I could not have sloped the wings or built the landing gear as I did without them. There are a couple of technic pieces hiding in Kestrel as well (the nose gear), and they're quite obvious on Skyraider. As Karim Nassar mentions, bricks don't always have to be oriented studs up. One of the easiest ways to get some of those odd studs-to-the-side angles is by using technic pieces. So basicly, think of technic pieces as little grey miracle workers.
To L3p or not to L3p
Raytracing: raytracing comes about as close to perfection as possible. Many a model has attained perfection only to loose it due to poor photography. Since part of the fun of building is sharing, you want what you share to look its best. Using Ldraw, L3p, and POV-Ray allows you to take the picture from any angle, with any lighting, always in focus, and using just about any piece that exists in any color and then some. The downside is that it's pretty boring and downright tedious to actually put a model into the computer. It takes about 2 hours to put an average sized model into the computer, more if you're new, and more if you have lots of wierd angles that need to be fine tuned, degree by degree. However, once that's done, pictures come out perfect every time.
That's odd...
Try using odd widths (1, 3, or 5 studs) for more blended curves. Offset studs with 1x2 w/1 stud on top plates, or use 3x2 plates with rounded side and key hole in them. Curves can look much nicer when they go from a width of 4 to 3 to 2 instead of just 4 to 2 studs. This also comes in handy for small models where 2 studs width can still be too big. A 2x10 instead of a 1x10 for Skyhop's tail, for instance, just wouldn't look right. A similar trick is employed on Triton's tail too.
Symmetry is for Geometry Class
Okay, now we're wandering into opinion territory (as if we weren't there already), but models do not have to be symmetric. Even the Blacktron Renegade, an official TLG model, had blatant asymmetry. Aside from TLG, there's the Millenium Falcon and the B-Wing (which I suppose are still TLG). One of my trademarks is to add a touch of asymmetry to just about everything I build. An antenna, a radar dish, an spotlight, or even larger weapons can look very nice when mounted on a single side.