At its heart, every figure tells a story. Before you worry about the "polishing" details, you first need to make sure that you're organizing the information to tell the story as clearly as you possibly can.
A classic book on the subject, which I have found very helpful, is Edward Tufte's "The Visual Display of Quantitative Information." This has a lot of great first-principles concepts for communicating information with figures, as well as a lot of great examples of figures that do well in doing so.
When you are working on your own figures, try playing with different arrangements and densities of information within a figure. Our first instincts are often wrong, so try splitting dense figures into separate charts and try combining figures together. Likewise, our eyes like to move in lines, so try shifting locations and orientations of objects in diagrams into different organization (e.g., vertical, horizontal, circular) to minimize overlapping lines and to bring the most important conceptual relationships into simple and salient geometric relations with one another. Color is another valuable dimension that you can use to organize relationships, whether to make certain things pop or to turn a bunch of individual lines into an organized gradient whose gist can be grasped with a glance.
Once you've got the core narrative elements of presentation sorted, then you can think about how you might want to polish the figure by adjusting fonts, line weights, colorblind compatibility, etc. Most modern software, however, starts you with pretty reasonable defaults for fonts and colormaps, so you may not need to worry much about those. Matlab, for example, has switched to parula, which is designed to work well for both colorblind viewers and greyscale printing.