When trying to design a sign-up page, I've generally started with the AIDA principle - grab their Attention, arouse their Interest, create Desire for the product, and present a clear call to Action. That said, that's only the first layer in this process. You can go as far down the rabbit hole as you like.
Des Traynor takes a good long dive into the aforementioned rabbit hole, and comes up with a number of insights. Worth reading for insights like:
Your app looks simple to use but is it powerful enough to handle my set-up? Does your software know about the taxes/rules about how invoices are handled in my country? Will I be your first serious customer, or do have experience dealing with firms of my size? Do other firms like mine use your software? How long have you been running? How do I know you won't wrap things up in a few months? Can I trust you guys? Can I talk to you guys? How do I know you are legit? Do you offer good support?
When you don't have the volume, go for the personal approach. When you can no longer go personal, then analyse the volume. At every step you need to ask yourself "Is every single thing on my website selling the product" and "Is there anything else I can include that will help".
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