The best principle to keep in mind is: keep it simple. Most self-published book covers fail because they are trying too hard. Even design professionals fall into the trap of trying to shoe-horn too much design into one composition.

I often tell students «your problem isn’t that you have poor ideas, it’s that you’ve got five ideas competing on the same page at the same time.»

Simplify. If in doubt, stick with typography. Make sure the typography is legible. Use your handwriting if your handwriting is decent. If not, use a font. Any tried-and-true standard face will do.

Many of the best book covers are as simple as could be. Peter Mendelsund.