How can I stop biting my nails?

Since I was quite young I have bitten my nails. It's not just a case of biting them when I'm bored or nervous, it has become a habit and I often do it unconsciously. I also bite them way past the quick, so that they often hurt or bleed. I have tried, in the past, to quit biting them, but I find using the bad-tasting nail polish or putting plasters over my nails does not deter me. Also, even if I do stop for a while, I will always get back into the habit so I'm not sure what to try now. Would it be worth visiting my doctor? And if so, what sort of things would they suggest? Are there are other methods that chronic nail-biters could try?