What are good principles for creating figures?
There are many books on this subject, as Peter Jansson points out, as it is something of an Art/Science in its own right. 

However, if you don't feel like buying and reading one or more books on the subject, here are a few basic things that I consider both important, and easy to follow:


Consistency: While people can generally argue for hours over the merits of one colour scheme over the other, what is more important is that you use a consistent one for all your figures. Same goes for fonts and line styles. Also, if you're going to use blue squares to represent some thing in a figure, be sure to use blue squares for nothing else throughout your entire paper.
Conciseness: Try to reduce each figure to making a single point, i.e. try to think in terms of "what is this figure trying to say". It's tempting to pack more and more data into a single figure, but in the end this will usually dilute the message. One figure, one statement.
Clarity: Once you've decided what it is that your figure will say, remove anything from it that does not contribute to this single statement. E.g. do you really need every data point/curve you've drawn? If your statement refers to part of a flow chart or class diagram, do you really need all the other, less relevant boxes/labels/methods there too? Also, if the salient feature of your figure is not immediately clear, don't shy away from adding an arrow or something to highlight it.
Completeness: Holding the balance to clarity's minimalism, make sure there is also nothing missing from the figure which is needed for the statement you want to make. Figure axes labels are a favourite. 


Funny how they all start with "C". This is not intentional.