Pre-existing book covers are your friend. Look at them, examine them and focus on studying books in your genre, but don’t be too disappointed if your self-publishing platform doesn’t support the trim size you inevitably fall in love with as a result (five by seven, five by seven!).
Make sure the cover correlates with the interior feel of the actual novel (and then dance about celebrating your own brilliance when you think of something that looks really cool).
Having twenty layers live at once in Photoshop will destroy your computer. No, really. The fan on mine has just… died, or something.
After spending four hours trying to align font at the bottom of your page to font at the top of your page, your eyes will go funny and everything will look wonky anyway.
The longer you stare at the block colour red, the more it begins to look like some whacky gradient.
Apparently due to variations in book binding the artwork for the cover will sometimes end up on the spine, and spine on the cover. You will be advised to avoid sharp lines and complex patterns, so naturally use them anyway.
You should (and probably will) print out your book cover and wrap it around some other less important novel just to see what it looks like on a bookshelf (mighty good, I tell you).
An actual, real life plastic ruler can be very helpful when determining layout spacing on your screen for the less technically gifted of us.
Asking (non-graphic designery) people their opinion on your artwork can be confusing. Just focus on the things they agree on, and ignore anything they don’t.
Making a book on a template before you have your final page count is just asking for trouble. You think you’re finished, then a last-minute cut adds or subtracts a page or two. New spine thickness, new document! Yeah!