Seth Godin suggests that maybe you shouldn't bother, since most of that traffic is random noise:
"'I'm just looking,' is no fun for most retailers. Yet they continue to pay high rent for high-traffic locations, and invest time and money in window displays. Very few retailers lament all the traffic that walks by the front door without ever walking in. A long time ago, they realized that the shoppers with focused intent are far more valuable. Smart retailers work hard to get focused people to walk in the door and to keep the riff raff walking on down the sidewalk.
Your website can do the same thing. In fact, you might want to make it more likely that bouncers bounce, not less, but only if those changes increase the results you get from the visitors you truly care about."
But when you think about it, Godin is exactly right: you're always going to get some visitors who came to your site by accident, and they left because you're not offering what they are looking for. Those people will never be customers. They'll never become members of your community. Nothing you do will ever change that. Far better, then, to spend your limited time finding ways to satisfy the people who do want to come to your site, and signal that desire by coming, staying for a while, and then coming back a second time.
A bird in the hand is worth two in the bush, and your bounce rate? That's the bush.