Let's consider purely technical aspects of icon design
1. Lines, angles
One of the vital elements of the image is the outline of the object, i.e. rough line, which limits the object from the rest of the image. When designing large-scale images, the designer occasionally thinks about highlighting of objects with additional outlines. This is not needed because of the size: even non-contrasted objects do not blur into a single whole. Pixel graphics is different. Two solutions are possible: either foreground and background colors have to be from opposite sides of the color wheel, or the object must be separated from the background by contrast outlines and shadows. I would like to dwell on lines in more detail.
If we think more to the big-size graphics, we can note that in order to define edges, we can use all (including the most complicated) angles, Bezier splines and edges. Anyway, the line will look ideal thanks to anti-aliasing. When decreasing the image scale to the small icon size, situation changes dramatically. When each particular pixel is equally valuable and can change the whole look of the icon, anti-aliasing is simply unreasonable. It means that you should consider the possible line slopes.
The angle you choose for the line, specifies the step of this line. Because each line is made of basic elements, the union of which determines its accuracy and esthetics.
For instance, a 18-degree angle contains many tiny 3-pixel lines combined with a 1-pixel downshift:
This is how almost each line looks like. Primary element with joining followed by second basic element. Unfortunately, not each angle makes the line look attractive and not messy.
For example, here are 25 and 20-degree slopes:
It can be easily seen which line is more attractive. The 25-degree angle produses a line consisting of equal 2-pixel primary elements. The 20-degree slope has a primary element containing of three lines: 3-pixel, 3-pixel and 2-pixel. The slope is more complicated, thus, when not combined by additional visual effects, it brings a feeling of untidiness: the basic elements have contrast which is too obvious to hide from the human eye.
Here we can come to the first conclusion. The most "correct" angles have to make as plain primary elements as possible. Thus, the perfect angles are 0 and 90 degrees. In them, 1-pixel primary elements are combined without any shift and produce smooth lines. Less perfect are angles which form primary elements of 1, 2, 3 pixel with 1-pixel downshift. Surely, the lines can not be smooth, but the human brain will process the image and provide to the viewer what you intended to show to him. Also, the mentioned angles can be considered correct (but at a stretch): slopes with basic elements made of unequal lines shifted by more than 1 pixel.
It is the time to discuss another problem. In the previous paragraph, I purposely defined slopes with basic elements of 1, 2 and 3 pixels as acceptable. Such tiny lines, especially when a great amount of them is combined, appear as a single whole. But what happens if we change the primary elements to 10 or 20 pixels?
Here is an example:
It can be clearly noticed that the line doesn't have integrity. It is not a continuous line anymore; it is a set of different lines, located near each other. Hardly any designer wants its composition to look that way. So, we have another rule: if you use minimal angles, which make long primary elements, it must be justified and employed with great attention.
And, finally, the third aspect I have to mention about lines. I did write that the basic element has to be as simple as possible.
For example, a 25-degree slope can be drawn in the two (sure more of them are possible) following ways:
In the first picture the primary element is a 2-pixel line. In the other case, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same slope but the image is different. The complicated basic element produced the line untidy.
This examples can be made for about any angle, so in your projects, try to simplify everything.
It can be said without an overstatement that the color is the main element of your work; improper or badly balanced colors can kill even the most creative idea. What can we write about the color in miniature graphics?
First, there is a certain rule: if the image looks poor when reduced to grayscale, then the colors that were selected are wrong. This rule is true not only for icons but for the whole graphic design.
Second, you will probably unable to apply common RGB. Sure, you can select internet colors only, but this limits you greatly. Gradients, shadows, dithering and many other tricks will not be available to you.
Third, try to increase the areas of "flat" color. The more evenly colored parts without diffusion and jumps your project has, the more clear it appears. Pictograms are too small and the over usage of special filters makes the image look dirty.
Fourth, gradients. This is a wonderful instrument which can change the look of any composition. Besides esthetics, gradients are an ideal solution to abolish the "broken line" look (lines with too long primary elements). But be careful when using them since too many gradients can kill the plain color, and be not suitable for the gif palette.
Fifth, shadows and highlights. All aspects of using shadows and flares in icon making are totally identical to the rules of the overall graphic design. The only ine I would like to mention: create everything yourself. Don't use filters, make all shadows and highlights in a different layer and then edit the opacity. When using effects, you almost never can predict what will it look like. It is unfavorable when you don't control the making of a composition. It is awful if you don't control it when creating miniatures.
And, lastly, sixth. Tinting is the main aspect of icon design, which greatly results the esthetic appearance of the project. If the blue human eye in the icon has the size of 1 pixel, increase the color saturation. For example, instead of 0, 131, 159 make 71, 195, 242. It is indifferent that the human eye never has such color; in the first case it will appear like a gray dot, but in the second case it will really be a blue eye.
Thoroughly control the picture, do not let the optical illusions to spoil your work. It is acceptable if objects is not perfect from the technical point of view, but the whole image must be perfect.
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 flares and nuances.
If I talked about something other than pictogram creation, this part wouldn't fit into a whole book, but the size of images restricts fonts too.
The resolution of letters becomes the main aspect as opposed to their look. Nearly in all cases, only if not the letters are the primary part of the image, the text size has to be reduced to the greatest extent possible.
In general, almost any design company has its exclusive font with small characters. This font can be made in a few hours. You can search the internet and collect your own library of very nice fonts. First of all, I would recommend you to get the DS Pixel and Seventen 7Vedi website and the complete set of fonts from the Lakmus Lab website.
If you don't need a new font, little typesizes of Arial and Verdana will work. As a last resort, you can draw the needed characters by hand.
There are not many rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters cannot use anti-aliasing. It turns the font into an incomprehensible set of pixels.
Though, there are no rules without exceptions and the 5-point Arial appears rather understandable.