If your firm has been around for long enough, then it will almost certainly have long-standing clients who repeatedly come back for changes, additions, and general maintenance to existing software, as well as new development. Those long-standing clients will also introduce new clients.
So it seems it is largely a matter of finding bootstraps to pull up on.
It is a matter of relationships.
You cannot control when a customer buys and you cannot create a sale.
All you can do is charge enough to stay in business and try to be worth your customers wanting to keep you in business.
Nobody cares if you start a business; you will be competing against companies whose customers pull their bootstraps; and starting out you will mostly deal with the inexperienced and the grifters.