Let's consider purely technical aspects of icon making
1. Lines, angles
One of the most important parts of the composition is the framework of the object, i.e. solid line, which separates the object from the background. When designing normal-sized graphics, the artist rarely cares about defining objects with additional outlines. This is not needed because of the scale: even low contrast objects do not mix into a single whole. Icon graphics is different. Two variants are available: either foreground and background colors must be from opposite sides of the color chart, or the object should be separated from the background by contrast outlines and shadows. I will discuss lines deeper.
If we return more to the big-size graphics, we can note that in order to highlight edges, we can use any (including the most complex) angles, Bezier curves and edges. Anyway, the line will look perfect thanks to anti-aliasing. When shrinking the graphic scale to the size of an icon, situation changes greatly. When each single pixel is vitally important and can change the overall appearance of the composition, anti-aliasing is just unreasonable. It comes that you should consider the possible line angles.
The slope you select for the line, determines the step for this line. This means that each line consists of primary elements, the union of which determines its neatness and esthetics.
For example, a 18-degree slope consists of many small 3-pixel lines connected using a 1-pixel downshift:
This is how almost every line looks like. Basic element with joining followed by another basic element. Unfortunately, not every angle makes the line look attractive and not messy.
For example, here are 25 and 20-degree angles:
It can be easily seen which line is more appealing. The 25-degree angle makes a line consisting of even 2-pixel basic elements. The 20-degree angle has a basic element containing of three lines: 3-pixel, 3-pixel and 2-pixel. The angle is more complex, thus, when not combined by other visual effects, it brings a feeling of untidiness: the basic elements have contrast which is too strong to escape the human sight.
Now we can come to the first conclusion. The most "appropriate" angles must make as plain primary elements as possible. Thus, the perfect slopes are 0 and 90 degrees. In those, 1-pixel basic elements are joined without any shift and produce smooth lines. Less ideal are angles which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Surely, the lines will not be even, but the human sight will process the picture and present to the user what you meant to show to him. Also, the mentioned angles can be considered acceptable (but at a stretch): slopes with primary elements consisting of different lines shifted by more than 1 pixel.
It is the time to touch another issue. In the last paragraph, I intentionally defined angles with basic elements of 1, 2 and 3 pixels as acceptable. Such tiny lines, especially when a large number of them is joined, appear as solid. But what do we see if we increase the basic elements to 10 or 20 pixels?
Here is an example:
It can be clearly noticed that the line doesn't have solidness. It is not a continuous line now; it is a combination of different lines, located near one another. Rarely any artist wants its creation to look inconsistent. So, we come to the second conclusion: if you use small slopes, which make long basic elements, it must be justified and used with great attention.
And, finally, the third aspect I have to tell about lines. I already wrote that the primary element should be as plain as possible.
For example, a 25-degree angle can be drawn in the two (of course more of them are possible) following ways:
In the first case the primary element is a 2-pixel line. In the other picture, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same angle but the look is different. The complex basic element made the line untidy.
Such examples can be made for about any angle, so in your projects, try to simplify everything.
It can be stated without an overstatement that the color is the main aspect of your work; improper or poorly matched colors can kill even the most creative idea. What can we say about the color in miniature graphics?
First, there is a certain rule: if the image looks bad when reduced to grayscale, then the colors that were chosen are wrong. This rule is valid not only for icons but for the entire graphic design.
Second, you will probably unable to use safe RGB. Of course, you can select internet colors only, but this limits you greatly. Gradients, shadows, blending and many other tricks will not be accessible to you.
Third, try to enlarge the portion of "plain" color. The more evenly colored parts without blurring and jumps your picture has, the more clear it appears. Pictograms are too small and the excess usage of special filters makes the picture look dirty.
Fourth, gradients. This is a great instrument which can change the appearance of any picture. Besides esthetics, gradients are an ideal way to get rid of the "broken line" effect (lines with too long primary elements). But be attentive when using them since too many gradients can kill the flat color, and be not suitable for the gif palette.
Fifth, shadows and highlights. All aspects of using shadows and flares in icon creation are completely identical to the aspects of the overall graphic design. The only thing I have to note: create everything yourself. Don't use effects, make all shades and flares in a separate layer and after that edit its transparency. When applying filters, you almost never can predict what the result will be. It is unfavorable when you can't control the process of creating of a composition. It is awful if you can't control it if creating icons.
And, finally, sixth. Nuancing is the main detail of icon creation, which greatly results the visual look of the composition. When the blue human eye in the image has the size of 1 pixel, increase the color saturation. For example, instead of 0, 131, 159 make 71, 195, 242. It doesn't matter 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 definitely be a blue eye.
Thoroughly control the picture, do not allow the optical illusions to spoil your design. It is acceptable if something is not consistent from the technical point of view, but the entire picture must be perfect.
As an example, I pictured the pack of juice located in front of me. The picture has flat color, gradients with broken lines, shadows and highlights and nuances.
If I talked about something other than miniature creation, this part wouldn't fit into an entire book, but the resolution of pictures restricts fonts too.
The size of symbols is the main issue as opposed to their fineness. Almost in all cases, only if not the letters are the primary part of the project, the text size has to be reduced to the largest extent possible.
In principle, virtually any design company has its own font with little characters. Such a font can be crafted in several hours. You can search the internet and collect your own collection of very interesting fonts. Primarily, I would recommend you to get the DS Pixel and Seventen 7Vedi webpage as well as the complete set of fonts from the Lakmus Lab website.
If you don't want a new font, tiny sizes of Arial and Verdana will do. As a last option, you can create the needed characters 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 transforms the text into an incomprehensible mixture of pixels.
Though, there are no rules without exceptions and the 5-point Arial looks perfectly understandable.