On a fixed annual budget, what’s an effective way to ensure that you have the budget flexibility you need to experiment with campaigns to test their effectivenes?

Check out industry reports that can help you predict query demand and set budget accordingly.
Set aside a percentage of the budget for use in experiments and response to market changes.
Send a supervisor a quarterly budget review as part of a request for additional testing allocations for the upcoming quarter.
Create a monthly or weekly sync with the finance team in order to foster collaboration, share news and ideas about campaigns, and encourage flexibility in the budget.