It may sound like a common sense thing, but I've lot count of the times I've followed a small business or self-published writer's twitter or facebook page, and seen my dashboard/wall suddenly flooded with nothing but advertisements for their product.
It's annoying, it's intrusive, and it almost always makes for an instant un-follow/un-friend.
Space it out. It's more effective to plant the seed and let people get curious than to beat people in the head with what you're selling.
