> * Some viral component so you do not have to spend money on ads for growth.
Indeed. For years our sales department was simply our customers. They'd call their friends and tell them they need to get our software, or when they switched jobs they'd work hard to get our software used at the new place.
Now we have a small but proper sales team. However word of mouth got us very, very far.
> * Sold as a subscription and only as a subscription.
This was a key point my boss realized early on as well. It's much easier for the accounting department to handle a fixed recurring cost, than a bi-annual huge expense or similar.
It also allows you to incorporate usage into the price, so the price scales with the customers' income, not unlike cloud offerings.
We have a small fixed monthly cost, and a variable cost (per invoice/order essentially). The variable price depends on volume, but it's very predicable and makes it so the customer doesn't have to worry too much if business is slow for a while.