Take a look at customers you have today and try to imagine what will happen with them five years from now. If you can imagine some of them grow much bigger than they are today, you're in great position. As they grow, you'll very probably grow along. Not only because your existing customer will have more money to spend and needs to fill, but even more importantly, because other companies will try to replicate their success. If you're able to show that you've contributed to their success, other companies will come knocking and they will be buying your products in order to be successful, too. On the other hand companies who aren't on the path towards the success are already lost. In five years they'll be either without money to pay for your services, without business to need your products, or acquired by some other company who will replace your software with the software the acquirer is using.
Of course, if it would be easy to predict future success of your customers, you could just buy their stocks and live happily after. But if you have some beliefs about the future of your target market, you should apply those beliefs to your customers, too, and discard customers who aren't aligned with the bet you're making. Actively shaping one's customer base is one of the hardest things to do for a company, as it requires to say no to people offering you money (and saying no to money pisses even more people than a usual no). But if you want to be a winner, there's no other way but to mingle with winners.