Additional resources I'd look at for planning my agency's selling strategies would the HubSpot Academy certifications on things like Frictionless Sales, Inbound Sales, Platform Consulting, Growth-Driven Design Agency, HubSpot Solutions Partner ... there may be a few others. There's a lot of gold in those courses on HubSpot's best practices. If I were setting up an agency today, that's where I'd spend a good chunk of time.
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@DVos what other context can you provide about your business and goals? There's a bunch of general advice out there on the web (like this article from Instapage and this article from Statusbrew), but I'm sure we'd be able to give you some more actionable ideas if we know:
What kind of services you offer (or would like to offer)
What kind of industries and clients you serve (or would like to serve)
What you're doing now that giving you trouble
What you're doing now that's working
Happy to share some deeper thoughts if you can tell us more about your current and desired states! Thanks!
Knowing what stage the business is at today will dictate which practices will make the most sense. In my experience, it's easy to overthink it in the early days.