1. Use a very expressive image for the cover. The cover is the first thing that potential customers see (in online catalogues). Make sure the image has a low-activity area where you can put the title.

2. Come up with a title that is expressive, exciting and easy to understand.

3. Choose a font that goes with your subject.
Some fonts

appear refined, such as Gardener or else Times New Roman. They can be used for a wide range of projects.
appear technical, like Upright or simply very matter of fact, like Arial. They are very good for all things technology related and for many scientific subjects.
seem very personal, if not intimate, such as the handwriting fonts like Roughly Write. They are well suited to everything that signals emotionality, like regional calendars (My Prairie Home).

4. Make sure there is a strong contrast between the background color and the font:

Choose white or a very light color as a font color when your background is dark and vice versa.
Make sure to place your words in a low-activity area on the image, and if necessary, choose a different image for your cover.

Calendar on the subject of C20