"I'm colourblind and often have LSD for breakfast"
Colours
Sometimes I wonder what the coders/designers are smoking when they're putting their effects together. Nice, or at least adequate effects are often ruined completely by the choice of the colour schemes. If you hear someone say that your production uses coder colours or the Dutch colour scheme, you should not be proud of yourself.
The four golden basic rules are:
1) THOU SHALT NOT USE THE BASIC RGB RED.
2) THOU SHALT NOT USE THE BASIC RGB GREEN.
3) THOU SHALT NOT USE THE BASIC RGB BLUE.
4) THOU SHALT NOT USE THE BASIC RGB RED, GREEN and BLUE COLOURS IN COMBINATIONS,
SO NO PURE PURPLE, PURE YELLOW or PURE BLUE-GREEN.
From hereon, they shall be known as coder colours. Avoid them if your design isn't really, really, really requiring them (still try to avoid them) - they are also considered as newbie colors as most tutorials use them for demonstrations as visual design is not their point. And of course, they will make everyone who sees your production puke.
If you want to make your demo red, green or blue, there's nothing stopping you of course, but add a bit of the other colours too, it makes it instantly more pleasing to the eye. Instead of 1.0f, 0.0f, 0.0f, use 0.9f, 0.2f, 0.15f or something.
Your next task is to go and find a picture you like. And no, I'm not talking about those cute Japanese girls on your hard drive, but real pictures. Like art and stuff. Doing demo coding/design is just as much visual activity as painting or doing "real" art, and believe it or not, but there are actually rules that say what you should and shouldn't do. Now, I'm not an artist, but I'm the type of a person who likes to visit art exhibitions and galleries and I would dare to suggest I know something about art. I suggest you go and visit your local art museum, it's both fun and educational (plus you will make an impression on a certain type of girls ;)). Many people would say it's boring, but try going alone and not with your mother and the chances are you'll like it.
Now that you've found a painting you like, examine it. Does it have mostly coder colours? If it does, you're beyond help and I suggest you keep to coding and maths and leave the design to other people. Now, think WHY do you like the picture. Is it the light? Is it the use of colours? Are they warm or cold? What sort of feelings does the picture bring out in you? Develop an eye for such things and think of how you can incorporate the same things to your demo stuff.
One point still to be made: I've criticized the rainbow-coloured things. And it's true, they mostly tend to look ugly as hell. Unless it's done intentionally by skilled people. For example, check out 232 by Ainc. It has VERY psychedelic colours. It's also a very, very cool demo and I recommend everyone download and watch it. But look closely, they don't have all the colours of the rainbow there either. Even though the demo is practically soaked in acid and trippy images, the colour scheme is still thought out. And while there are some small flashes of pure RGB colours, they don't dominate.
Composition
The things about the choice of colours I've talked applies to composition as well. It really does matter where on the screen you put your stuff. To make a really crude example, just think what your favourite demo would look like if all the effects were on the left side of the screen. Again, watch and learn. Try to make it look alive. A good and simple tip is not to put the effects or rotating objects or whatever you have at the exact center of the screen. If done properly, it will bring variation to the visual display. Also, move them around a bit.
And most importantly, watch demos. A lot of demos. Demos from the Eighties right up to the latest parties. Think why they look good (if they do). Try to see past the "OMG there's a whizzler-mapped swinging thingamajic with shiny reflections!!!" and look at the big picture. Often you can see things that could easily be improved with a little thought.
There are countless books on the subject, so I'm not going to go in depth here. I admit that I don't know the theory either, but I can see what looks good and what does not. It's all a matter of learning how to look at things.
Meaning of the visuals
When you're planning the visual outlook of your demo, keep in mind that people react more strongly to the things they are familiar with. And reactions are exactly what you want if you're doing a well-designed demo. You're not only showing off your coding skills, you're building a visual display. I think this is best illustrated by an example:
Suppose you have a routine that rotates and envymaps 3d-objects. Most likely you do. What kind of objects would you want to show? Everyone has the morphing plasmaball, so that's probably not a good idea. Duck.3ds or chrome.3ds might be good as a joke, but nothing more. So, being the coder, you settle for a... Haujobb-styled spikyball object. It's not that bad a choice, if it otherwise fits your visual design. But what if you had chosen something people are familiar with? Like a head of a clown? Or Beethoven, as the new Condense demo has. It's much easier to make people go "oooh" when you rotate a Beethoven than when you rotate "just a head object".
Let's continue the example. Now, suppose you improve your routine and add the ability to morph from and object to another. You also include code that allows to you push spikes out of the object. Being the smart designer, you know you shouldn't show the spikes instantly, but only after the effect has been running a while. Okay, now an exercise. Which object morph chain would be better in your opinion?
A) Your group's logo - a cube - a torus knot - a duck - Beethoven - an airplane - a ball (the spikes appearw) - a Haujobb-syled thingamajic - a human head.
B) Your group's logo - a human head - a butterfly - a cat head - a hissing cat head - Albert Einstein - George W. Bush - (the spike appear) - Adolf Hitler - a skull.
Thought so.
Symbols
This is an extension to the previous point. Use things that people are familiar with, and use things that actually have a meaning. There are things in the collective unconsciousness of people that can be used to your advantage. Examples might be the cross, the Hindu god-sign, a peace sign, a rose and a pentagram. They actually mean things to people. Just keep in mind that if you make a burning cross on your demo, you might offend some people.
Okay, Now I've Got This And This And This And This, What Next?