There are 3 things you must never ever do on your blog...
1. Don't stray from your niche. Unless the topic of your blog is catholic culinary doctrine, feline transportation techniques or inside-the-beltway gossip, do not pad out your blog with posts about Fish on Fridays, How to keep a cat happy, or Truth and the Bush Regime. Your readers come to your blog because they want to read about the main subject. That's why you will never find me writing about ditsy subjects like goldfish, carrots, or sunsets.
2. Don't piss off your most loyal readers by criticizing their blogs.
3. Never, never, never write a post purely for the purpose of entering one of those tantalizing blog competitions, even if there is a chance of winning a hundred bucks.