I've worked extensively in influencer marketing, and one hugely important thing to remember (that a lot of people don't) when leveraging someone's existing audience is that it isn't your audience.
What I mean by this is that for your campaign to succeed and hit the right notes, you have to let your influencer have creative freedom to do their own thing within your brief. The audience you're trying to reach follows this influencer because they love their content, and often because of their authenticity and relatability.
As soon as a brand gets heavy-handed and tries to strictly control the content being produced, the audience will see straight through it and it may end up doing more brand damage than good.
Of course this isn't to say that you shouldn't have a good idea of what the content you want produced on your behalf, but make sure from the very first step that it's in true collaboration with the influencer. They know their audience best and what they'll enjoy watching.
This process can be a lot of fun if you're trusting and open-minded with your chosen influencer(s), but a lot of it comes down to choosing the right person, not just chasing the highest follower numbers. Look at other brand collabs they've done. Assess what you like about them, and call them up and have a chat. Is it someone you think you can work with?
A final piece of advice is that I'd always try and find those gems of influencers who have large-ish audiences but don't use agents. You can knock them down 50% from their original quote most of the time (when done sensitively and respectfully), whereas agents rarely ever budge.
