Whatever outreach you used to win your new client can also be used to keep them.
TL;DR—please do not stop marketing after you close the sale.
Consider using all of your marketing skills and methods
Any and all things at your disposal
Increase attendance at the onboarding
Write clear agendas with clear takeaways
Tell them what they should expect next
At the NEXT meeting
Where you will continue to access the things from them
That allow you and your team
To do what they hired you to do
Access to things like: their problems, their process, their insight into their customers
Heck, their usernames and passwords!
Assuming your client is in, say, the technology sector.
After the sale:
Getting your clients to participate
And see the value of the time you have together
In other words
All the good stuff you used
To close the sale
Can be used
To keep the sale.