Don't Bash Competitors or Past Customers: The community is not place to air your dirty laundry, or resolve customer service issues. Take those off-line. Talk up your competitors strengths, not their weaknesses.

Be Open to Feedback: If someone from community buys your product, be open to feedback about it. This is free market research and usability testing for you. If someone posts a complaint about your product, publicly thank them for taking time and request an off-line way to follow up. Keep in mind everyone is watching how you handle yourself - taking high road is only way to guarantee future sales here. Keep in mind this community is already connected, and word will get around regardless.

Offer Group Discounts: Any large web community will likely have common needs. Offer a discount on products or services if community can arrange a group buy.

Offer to Support Community: Most email lists or discussion forums have usually have costs associated with them that either one person is paying for, or community is paying for. Before they get mad at you for making money off of their efforts, offer to help pay for those costs - sponsor a month's worth of hosting, or donate a product to be auctioned off. Other advertising channels will cost you money, don't assume this is a free ride.

Be Involved: All too often communities see businesses post once then never return. Don't do that. If you're serious about helping this community out, become part of it. Take part in off-topic threads. Give advice that doesn't include a pitch for your products or services. Get to know people that make up this community on a personal level - don't just view them as your "target market".