Asking for 2 years of income is a good general rule.
Also factor in growth rates over the next 2 years. Estimate growth based on past growth, not on "well I have this new feature that should double traffic.
Factor in any assets you may have, this includes but is not limited to: - Newsletter subscribers - RSS Subscribers - High search engine rankings - Good domain name - Industry leader
Think about the sentimental attachment. This is worth something, figure out your price.
Finally forget about the lump sum of money. Its sexy to think about a big pile of cash, but you have something in your hands that is making a consistent profit. Think in 2 years if you'll still be happy with this decision.
Take this advice from someone whose sold a website that has seen enormous growth since I sold it. Its amazing to see something you've made become huge, but also a little sad thinking it could still be yours.
Good luck, you're in a good position. A buyer approaching you means you have something they really want. Don't forget that!