Let's consider purely technical aspects of icon creation
1. Lines, slopes
One of the most important parts of the composition is the framework of the object, i.e. rough border, which limits the object from the rest of the image. When designing normal-sized graphics, the artist occasionally cares about defining objects with additional lines. This is not needed because of the scale: even low contrast objects do not blend into a single whole. Pixel graphics is other story. Two variants are available: either foreground and background colors must be from different sides of the color wheel, or the object should be divided from the background by contrast outlines and shadows. I will dwell on lines in more detail.
If we think more to the big-size images, we see that in order to highlight contours, we can use all (even the most complex) angles, Bezier curves 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 every particular pixel is equally valuable and can ruin the whole look of the icon, anti-aliasing is just unreasonable. It means that you should think about the possible line slopes.
The slope you choose for the line, determines the step of this line. Because every line consists of primary elements, the combination of which determines its neatness and esthetics.
For example, a 18-degree angle contains many small 3-pixel lines connected using a 1-pixel downshift:
This is how nearly every line looks like. Basic element with joining followed by second primary element. Unfortunately, not every slope makes the line look attractive and not annoying.
For example, here are 25 and 20-degree angles:
It can be easily seen which line is more attractive. The 25-degree slope makes a line containing equal 2-pixel basic elements. The 20-degree slope has a primary element consisting of three lines: 3-pixel, 3-pixel and 2-pixel. The angle is more complicated, therefore, when not supported by other visual effects, it brings a feeling of untidiness: the primary elements have contrast which is too strong to escape the human sight.
Here we can see the first conclusion. The most "correct" slopes have to make as simple primary elements as possible. Thus, the perfect angles are 0 and 90 degrees. In them, 1-pixel primary elements are joined without downshift and form even lines. Less perfect are angles which form basic 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 user what you meant to show to him. Also, the following slopes can be considered acceptable (but at a stretch): slopes with basic elements made of unequal lines shifted by more than 1 pixel.
It is the time to move to another problem. 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 amount of them is combined, appear as 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 obviously noticed that the line doesn't have solidness. It is not a single line now; it is a set of several lines, situated near each other. Hardly any designer wants its composition to look inconsistent. So, we have another rule: if you use small angles, which produse long primary elements, it has to be reasoned and employed with maximum caution.
And, finally, the third aspect I have to tell about lines. I already wrote that the basic element has to be as plain as possible.
For example, a 25-degree angle can be drawn in the two (sure more of them are possible) following ways:
In the first case the primary element is a 2-pixel line. In the other case, it is made of three elements: 3-pixel, 2-pixel and 1-pixel. The same angle but the look is different. The complicated basic element produced the line messy.
Such examples can be made for about any angle, so in your works, make sure to simplify as much as possible.
It can be stated without an exaggeration that the color is the main element of your work; improper or badly balanced colors can kill even the best idea. What can we say about the color in miniature graphics?
First, there is a certain rule: if the image looks poor when reduced to grayscale, it means the colors that were used are incorrect. 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 may use internet colors only, but this ties you hand and foot. Gradients, shadows, blending and many other effects will not be available to you.
Third, try to increase the portion of "flat" color. The more evenly colored parts without blurring and jumps your composition has, the more clear it appears. Pictograms are too small and the excess usage of complex filters makes the picture look dirty.
Fourth, gradients. This is a great instrument which can improve the look of any picture. Besides visuals, gradients are a perfect solution to get rid of the "broken line" effect (lines with too long primary elements). But be careful when using 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 shades and highlights in icon creation are totally identical to the rules of the general graphic design. The only thing I would like to note: create everything by hand. Don't apply effects, make all shades and flares in a different layer and after that edit the opacity. If using effects, you almost never know what the result will be. It is bad when you can't control the process of creating of a project. It is worse if you can't control it when creating icons.
And, finally, sixth. Nuancing is the key aspect of pictogram creation, which largely results the esthetic appearance of the composition. When the green human eye in the icon is sized 1 pixel, increase the color saturation. For instance, instead of 0, 131, 159 make 71, 195, 242. It doesn't matter that the human eye can't have such color; in the first case it will appear like a gray dot, but in the second example it will really be a blue eye.
completely control the picture, do not let the optical illusions to ruin your design. It is OK if something is not consistent technically, but the whole composition must be perfect.
As an example, I pictured the packet of juice located before me. The image has flat color, gradients with broken lines, shades and highlights and nuances.
If I wrote about something bigger than icon creation, this chapter wouldn't fit into an entire book, but the size of pictures ;limits fonts too.
The resolution of symbols becomes the leading issue as opposed to their beauty. Nearly in all cases, only if not the letters are the main part of the project, the text size has to be decreased to the greatest extent possible.
In principle, almost any design company has its unique font with little letters. This font can be crafted in a few hours. You can search the internet and make your own collection of very interesting fonts. Primarily, I advice you to download the DS Pixel and Seventen 7Vedi website and the whole set of fonts from the Lakmus Lab website.
If you don't want a new font, little typesizes of Arial and Verdana may work. As a last resort, you can draw the necessary characters by hand.
There are quite a few rules there. First, letters 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 unrecognizable set of pixels.
However, there are no rules without exceptions and the 5-point Arial looks rather readable.