I am a developer and usually writes web scrapers or automation tools most of the time beside typical web development. A couple of things I did and worked for me.

- I offer them more than what they expect. At times scraping additional info which I think is useful but they did not realize I extract that too. Sometimes they ask for the script or data, one of them and I just offer them both and they appreciate it lot. Though that script is not helpful for them and they eventually come to me but it's just increase their trust.

- It sounds silly and dangerous but often I don't ask advanced payment from clients and prefer to show off some skills, mostly it was quite helpful and they worked me on other projects as well.