2) Try to keep it 10 words and under.
3) Do not use names of characters. That's a waste of your space.
4) Don't repeat the title of the book. It's already there for the reader to see.
5) Tell something about the character but keep it short. A lone cowboy... A handsome sheriff...
6) Focus on the main thrust of the book, whether it's romance, action, mystery, etc.
7) A short blurb from another writer can sometimes work even better. You can use a shorter quote from the blurb if the one an author sends you is a little long.
8) You are better off with no tagline rather than a bad tagline. A good tagline should help you sell books.
9) Work as much information as possible in those 10 words. You'll notice in Witch Got Your Tongue? that the series name, A Tongue Tied Witch, is worked into the tagline. In Tinseltown, you know it's a romance, suspense, historical and the location just from the tagline.
10) Now, last but not least...there are always exceptions.