Key
Dictionary definitions
A map key or legend essentially acts like a dictionary, providing definitions for symbols.
Organize your definitions as a dictionary: first state the term needing definitions, then state the definition. In legends, this translates to symbol on left, label on right. The symbol-labels should align horizontally on a row. Columns of symbol-label pairs should also be aligned: the symbols vertically, the labels left-justified.
Tobler meets Sullivan
When designing a legend, consider following a mixture of Tobler (near things are more related than far things) and Sullivan (form should follow function).
Organize your symbol-label pairs into groups that will help someone use your map. Identify the criterion or criteria that best relates the map features. Is it by type hierarchy (these thing are all kinds of roads), geometry (these things are all line features), or purpose of the map readers (there things are all about driving)? Keep in mind that there may be levels to your legend’s organization (first purpose, then geometry, then type hierarchy).
Define in context
Symbols on a map usually appear in context with other symbols and a visual background, yet legends often define symbols isolated from this graphical context.
Consider designing your legend so that symbols appear in graphical context. For example, show how the road looks against the background where you typically find the road. Or create more expressive graphical legends that act as diagrammatic models. For example, show roads, creeks and bridges together.