Firstly, you have to create your network. This is easy using the intuitive Ning interface. It's all fully customisable and doesn't require any programming knowledge. However if you do have some programming knowledge, use it; sites and networks that look unique will stand out and gain more members. Members are the most important ways to make your communities thrive; gaining new ones is your constant goal.
Once you have created a gorgeous, unique network, you need to get the word out. You need people to visit your site any way you can. Research SEO to get your site high up on Google, and try and get as many inbound links as you can. If you know friends with blogs or websites, now is the time to call in those favours. Also contact other bloggers whose theme or interest is similar to the one on your site to see if they will link to you in exchange for you linking to them. Make your blog visible across the internet— the more it's seen, the more people will visit.
Don't just stick to the internet, though. Try and find real-world ways to get new members to your site. If you can entice someone onto using the internet by making them interested in your site, you'll probably have a member for life. Try and find ways to promote your site in real-life situations that link to it. If your site is about food, hand out flyers in cafes. If your site is about exercise, have a t-shirt with the URL on made up and wear it whilst jogging or at the gym.
Once your network is out there a bit more, your community can begin to grow. Now it's time to start making connections. Use other networking sites such as Twitter and Facebook — search for tweeters with similar interests and strike up a conversation with them, linking to your network on your Twitter stream. Start a fan page for your network on Facebook and spend a little money on Facebook adverts.
By now you should have members. Now you've got to keep them, and get them to bring friends! Show them that your site is dynamic and interesting. Make sure that your content is interesting and try and keep discussions in any forums you may have fresh. Send a weekly or monthly e-mail to all members that highlight the most popular discussions, events or content. This will help new members catch up on what's been going on and give inactive members a poke to have another look on your site to see what's going on.
Once your community has started growing, you simply need to do whatever you can to keep the momentum going. You’ll have a thriving community in no time.