Automate as much as possible. Reduce the time you need per customer, so if you record how much time you need for current customers -- then multiply that out. If you had 2x the customers would you be able to support them properly? How about 10-50x?

Is there anything you can do to make the existing customers happy? If you keep concentrating on them, more will follow.

Just take it slowly... don't do massive advertising/marketing spends all at once. Control the rate of new customers you get.

Each time you grow you'll probably find issues, so best to do it slowly... otherwise everything could fuck up.

It's tempting to go 'oh things are working now, let's throw $20-80k at advertising tomorrow and see what happens!'

Nothing worse than having 20,000 angry customers.