I know this sounds obvious, but "word of mouth" is massive. It works particularly well when your product is something that other companies are forced to interac

I know this sounds obvious, but "word of mouth" is massive. It works particularly well when your product is something that other companies are forced to interact with. So for a [bad] example let's say you offered FTP as a SaaS solution. Let's say your client is "client.com" and you are "ftpsaas.com" when your client buys your services, instead of letting them do "ftp.client.com" you FORCE them to do "ftp.ftpsaas.com." That way when other companies interact with client.com they also can see that your service ftpsaas.com is involved. This makes you a "known name" within that area, it also often encourages them to check you out to see /why/ client.com thinks you are such a good idea (assuming client.com is someone they trust). So if you have a single good client you almost feed off of their reputation to build your own. As I said, this only works with SaaS which requires external parties to connect to you or for you to connect with them. If you offer like accounting services for example the same effect doesn't exist.