post regularly on other boards and put the link to yours in you signature.
But if your site is about BMX bikes, then doing this on sites about BMX bikes or about other bikes will have better results. Your signature is almost worthless on a site dedicated to other topics. For instance, this is a programming site, so your signature is about as valuable as standing on the street corner handing out paper flyers. You need to visit the bike and BMX sites. That is where the audience would be.

Of course, trying to create a site on mutliple, distinctly different topics would be hard. For example, people who are interested in soccor would probably not be interested in PHP or on-line gaming. PHP people are probably not interested in soccor or on-line gaming, etc. Who are you trying to please with your site? The people interested in all three? That is probably a very small group of people.