Let's consider purely technical details of icon making
1. Lines, angles
One of the most important parts of the image is the framework of the object, i.e. rough border, which limits the object from the rest of the image. When designing large-scale images, the artist rarely thinks about highlighting of objects with additional lines. This is not needed because of the scale: even low contrast objects do not mix into a single whole. Pixel graphics is different. Two solutions are available: either object and background colors have to be from opposite sides of the color wheel, or the object should be separated from the background by visible lines and shadows. I would like to discuss lines deeper.
If we return again to the big-size graphics, we see that in order to highlight edges, we can use all (including the most complicated) angles, Bezier splines and borders. In any case, the line will appear ideal due to anti-aliasing. When decreasing the image scale to the small icon size, situation changes dramatically. When each particular pixel is vitally valuable and can change the whole appearance of the composition, anti-aliasing is simply unreasonable. It means that you should consider the possible line slopes.
The angle you select for the line, determines the step of this line. This means that every line consists of primary elements, the combination of which determines its neatness and visual appeal.
For example, a 18-degree slope contains many small 3-pixel lines combined with a 1-pixel downshift:
This is how nearly each line looks like. Primary element followed by joining and second primary element. Unfortunately, not each slope makes the line look neet and not messy.
For example, look at 25 and 20-degree slopes:
It can be easily seen which line is more attractive. The 25-degree slope produses a line containing equal 2-pixel basic elements. The 20-degree slope has a basic element consisting of three parts: 3-pixel, 3-pixel and 2-pixel. The slope is more complex, therefore, when not supported by other visual effects, it brings a feeling of messiness: the basic elements have contrast which is too obvious to hide from the human eye.
Here we can see the first conclusion. The most "correct" slopes have to make as plain basic elements as they can. Thus, the ideal angles are 0 and 90 degrees. In those, 1-pixel primary elements are joined without any shift and form even lines. Less ideal are angles which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines can not be smooth, but the human sight will process the image and provide to the viewer what you intended to show to him. Also, the following slopes can be considered acceptable (but at a stretch): angles with basic elements made of unequal lines shifted by more than 1 pixel.
Now we can touch another issue. In the last paragraph, I intentionally defined angles with primary elements of 1, 2 and 3 pixels as acceptable. Such small lines, especially when a great amount of them is joined, appear as solid. But what do we see if we change the primary elements to 10 or 20 pixels?
Here is an example:
It can be obviously noticed that the line doesn't have solidness. It is not a single line now; it is a combination of different lines, situated near each other. Hardly any artist wants its creation to look that way. So, we come to the second rule: if you use minimal angles, which make long basic elements, it must be reasoned and used with maximum attention.
And, finally, the third thing I have to mention about lines. I already wrote that the primary element should be as plain as it can.
For example, a 25-degree slope can be drawn in the two (of course more of them are possible) following ways:
In the first case the basic element is a 2-pixel line. In the other picture, it is made of three lines: 3-pixel, 2-pixel and 1-pixel. The same angle but the look is different. The complex basic element produced the line untidy.
This examples can be made for almost any angle, so in your projects, try to simplify everything.
It can be stated without an overstatement that the color is the leading element of your work; improper or badly balanced colors can ruin even the best idea. What can we write about the color in miniature graphics?
First, there is a clear rule: if the picture looks bad when reduced to grayscale, it means the colors that were chosen are incorrect. This rule is true not exclusively for icons but for the entire graphic design.
Second, you will probably unable to use safe RGB. Sure, you can select GIF colors only, but this ties you hand and foot. Gradients, shadows, dithering and many other effects will not be available to you.
Third, try to increase the areas of "plain" color. The more evenly colored areas without blurring and jumps your picture has, the more neat it appears. Icons are too small and the excess usage of complex effects makes the picture look messy.
Fourth, gradients. This is a wonderful tool which can change the appearance of any composition. Other than visuals, gradients are an ideal way to abolish the "broken line" effect (lines with too long basic elements). But be attentive when applying them because too much gradients can ruin the flat color, and be unable to fit into the gif palette.
Fifth, shadows and highlights. All aspects of using shades and highlights in icon making are totally the same to the rules of the overall graphic design. The only thing I have to note: draw everything yourself. Don't use effects, make all shades and highlights in a separate layer and after that edit its transparency. If applying filters, you rarely know what the result will be. It is unfavorable when you don't control the making of a project. It is awful if you don't control it when creating icons.
And, lastly, sixth. Tinting is the main detail of icon creation, which largely affects the esthetic look of the image. If the green human eye in the icon is sized 1 pixel, increase the color intensiveness. For example, instead of 0, 131, 159 make 71, 195, 242. It is indifferent that the human eye can't have this color; in the first example it will appear like a gray dot, but in the second one it will really be a blue eye.
Thoroughly control the image, do not let the optical illusions to spoil your work. It is acceptable if something is not perfect from the technical point of view, but the whole composition must be flawless.
As an example, I drew the packet of juice standing in front of me. The picture has plain color, gradients with broken lines, shadows and highlights and tints.
If I talked about something other than miniature design, this part wouldn't fit into a whole book, but the size of images restricts fonts too.
The resolution of symbols is the leading aspect as opposed to their fineness. Nearly in any case, only if not the letters are the main part of the project, the text size must be decreased to the largest extent possible.
In general, almost any graphic company has its own font with small characters. This font can be crafted in several hours. You can browse the web and make your own collection of very nice fonts. First of all, I would recommend you to download the DS Pixel and Seventen 7Vedi website and the entire series of fonts from the Lakmus Lab website.
If you don't want a new font, little sizes of Arial and Verdana may do. As a last option, you can draw the needed letters yourself.
There are quite a few rules there. First, letters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters should not use anti-aliasing. It turns the text into an unrecognizable set of pixels.
Though, there are no rules without exceptions and the 5-point Arial looks rather understandable.