Let's discuss purely technical details of icon design
1. Lines, slopes
One of the vital parts of the image is the outline of the object, i.e. solid linear marking, which limits the object from the rest of the image. When working with normal-sized images, the artist rarely thinks about highlighting of objects with additional lines. This is unnecessary because of the scale: even non-contrasted objects do not mix into a single whole. Pixel graphics is other story. Two variants are possible: either object and background colors have to be from different sides of the color wheel, or the foreground must be separated from the background by contrast lines and shadows. I would like to dwell on lines in more detail.
If we return again to the large scale graphics, we can note that in order to highlight edges, we can use any (including the most complex) angles, Bezier curves and edges. In any case, the line will look perfect thanks to anti-aliasing. When shrinking the image scale to the size of an icon, situation changes greatly. When each single pixel is vitally valuable and can change the whole look of the icon, anti-aliasing is just unreasonable. It means that you should consider the possible line slopes.
The angle you choose for the line, specifies the step for this line. This means that every line consists of basic elements, the combination of which determines its accuracy and visual appeal.
For instance, a 18-degree slope contains many tiny 3-pixel lines combined with a 1-pixel downshift:
This is how nearly each line looks like. Primary element followed by joining and second basic element. However, 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 appealing. The 25-degree angle makes a line containing even 2-pixel basic elements. The 20-degree slope has a primary element containing of three parts: 3-pixel, 3-pixel and 2-pixel. The slope is more complex, thus, when not combined by other visual effects, it creates a feeling of messiness: the primary elements have contrast which is too obvious to hide from the human sight.
Here we can see the first rule. The most "correct" angles must make as plain primary elements as possible. Thus, the ideal angles are 0 and 90 degrees. In those, 1-pixel primary elements are joined without any shift and form smooth lines. Less perfect are slopes which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines can not be smooth, but the human brain will process the image and present to the viewer what you meant to show to him. Also, the mentioned angles can be considered acceptable (but at a stretch): slopes with basic elements consisting of different lines shifted by more than 1 pixel.
It is the time to discuss another issue. In the previous paragraph, I purposely defined angles with primary elements of 1, 2 and 3 pixels as correct. Such small lines, especially when a great number of them is joined, look like a single whole. But what do we see if we increase the basic elements to 10 or 20 pixels?
Here is an example:
It can be clearly seen that the line loses its solidness. It is not a single line now; it is a set of several lines, located near one another. Rarely any designer wants its creation to look that way. Thus, we come to another rule: if you use minimal angles, which make long primary elements, it has to be justified and used with maximum attention.
And, finally, the last aspect I wanted to tell about lines. I did write that the primary element should be as simple as it can.
For example, a 25-degree angle can be produced in the two (of course more of them exists) following ways:
In the first case the primary element is a 2-pixel line. In the second case, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same angle but the image is different. The complicated primary element made the line untidy.
This examples can be made for almost any slope, so in your projects, make sure to simplify as much as possible.
It can be said without an exaggeration that the color is the main element of your work; improper or badly matched colors can ruin even the best idea. What can we say about the color in miniature graphics?
First, there is a certain rule: if the picture looks poor when converted to monochrome, it means the colors that were chosen are incorrect. This rule is valid not only for icons but for the entire graphic design.
Second, you will probably unable to apply safe RGB. Sure, you may use internet colors only, but this ties you hand and foot. Gradients, shadows, dithering and many other tricks will not be available to you.
Third, try to increase the portion of "flat" color. The more consistently colored areas without diffusion and jumps your composition has, the more clear it looks. Pictograms are too small and the excess usage of complex effects makes the image look messy.
Fourth, gradients. This is a great instrument which can improve the look of any project. Besides visuals, gradients are an ideal solution to get rid of the "broken line" look (lines with too long basic elements). But be careful when applying them since too much gradients can ruin the flat color, and be not suitable for the gif palette.
Fifth, shadows and highlights. All rules for using shadows and highlights in icon making are completely identical to the aspects of the general graphic design. The only thing I would like to note: draw everything by hand. Don't use filters, make all shades and highlights in a separate layer and after that edit the opacity. When using filters, you almost never know what the result will be. It is unfavorable when you can't control the process of creating of a composition. It is worse if you can't control it if creating miniatures.
And, lastly, sixth. Tinting is the main aspect of icon design, which largely results the esthetic appearance of the project. When the green human eye in the image has the size of 1 pixel, increase the color intensiveness. For instance, instead of 0, 131, 159 choose 71, 195, 242. It is indifferent that the human eye never has such color; in the first example it will appear like a gray dot, but in the second case it will really be a blue eye.
Thoroughly control the image, do not allow the optical tricks to spoil your work. It is OK if something is not perfect from the technical point of view, but the whole image must be perfect.
As an example, I drew the pack of juice located in front of me. The picture has plain color, gradients with broken lines, shadows and highlights and tints.
If I wrote about something bigger than miniature creation, this part wouldn't fit into a whole book, but the resolution of pictures ;limits fonts too.
The size of letters becomes the main aspect as opposed to their beauty. Almost in any case, unless the letters are the primary part of the image, the font size has to be reduced to the largest extent possible.
In general, almost any graphic studio has its own font with little characters. This font can be made in a couple of hours. You can search the web and collect your own library of very nice fonts. Primarily, I would recommend you to get the DS Pixel and Seventen 7Vedi website as well as the complete series of fonts from the Lakmus Lab website.
If you don't need a new font, tiny typesizes of Arial and Verdana may do. As a last resort, you can draw the needed characters yourself.
There are quite a few rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters should not use anti-aliasing. It turns the font into an incomprehensible set of pixels.
However, there are no rules without exceptions and the 5-point Arial looks rather readable.