1. Involve other Businesses
Invite other local businesses that are not in direct competition with your business to participate in your open house. Offer them the opportunity to open a booth in the parking lot for free in exchange for them contacting every customer they have on their emailing list. A restaurant would be a good business to invite to ensure your visitors are well fed.
2. Invite Performance Groups
Invite entertainment groups like martial artists or danceers to come and do a performance for your open house. Most will do it for free, and you want to invite these groups because not only will you bring in more people to your open house through the groups themselves, but also through the families and friends that will come to support them.
3. Ask for Volunteers
Ask your close friends, family, and neighbors to help participate at the event. Most people are more than willing to help out, especially if it's for a charity.
4. Send a Press Release
Let every news station and newspaper know about your event so that it can get publicized for you. The more sources of publicity the better, especially if it's free.
5. Send Invitations via email/Facebook
Send an invitation to every person you have on your business' mailing list and ask them to forward the invite to all of their friends. Another way you can do this is to set up a Facebook event about your open house. This is effective if you have many loyal customers who are already fans of your Facebook page. If not, it only takes a few minutes to set up an event, but don't expect much of a response this way.
Choose activities that allow the visitor to walk away with something that has your logo and business information on it. You'll get more of a response the easier you make it to win. You can also choose to have one or two major prizes that you can readily donate based on your business entity.
To achieve maximum success with your open house, be sure to plan well in advance and think through what your visitors will want to engage in when they show up. Avoid "down time" as much as possible. Make it difficult for your visitors to leave because they're enjoying themselves so much.