Clans usually promote themselves by playing clan wars, being active and communicating with people. It usually works naturally.
People don't notice clans which are not active and usually want to join the most active clans.
By just asking people to join your clan, you actually achieve only more members who are not part of the clan, just for quantity.
The best way is to be active and to ask people you often play with to join your IRC channel, that's how they can get closer to your clan and be better known to the members.
New clans also should happen naturally. The reason to create a new clan should be creating a team of people you enjoy to play with. Not just, "Oh, I can create a clan, so let's find people by asking them to join it".
That's my opinion.