Websnark has a lengthy post of advice to aspiring bloggers and critics. He’s specifically talking about webcomic reviewing, but his recommendations are easily generalizable, with lots to learn and think about. My favorites:

Accept that not everyone is going to agree with you.

Any time you figure it’s self evident? It’s not.

Don’t argue your point on that webcomic’s forums. (Heck, I’d say anywhere.)

Don’t make every conversation you’re in be about you.

Be prepared for thunderous silence.

There’s a lot more at the link, with lots of explanation.