Amy Stone at Absolute Write tells you 'mistakes' not to make, by which she means do make these mistakes. She really pulls no punches:

Your novel is your brainchild, so why would you risk infecting it with outside influences and potentially damaging information? Writers' conferences, literary groups, message boards, writers' blogs, agents' websites… poisonous distractions all. You may think that the more you know about the process and what agents and publishers are seeking, the more likely you'll be to succeed. Wrong. The more you learn while in the throes of writing, the more likely you'll be to second-guess every right decision you've made. Next thing you know, you're questioning the title of your book, rewriting the main character, and changing the voice from third-person to first. Then, just when you're ready to explode with indecisiveness, you land on a BBS for frustrated novelists and discover you're not the only one going nuts all alone with a rapidly unraveling piece of literary work. What a revelation. You begin writing again, and you do some of your best work. Too bad they're only messages on a bulletin board and you posted them under a pseudonym.

Prepare to feel ashamed, but enlightened....