It depends on what you’re wanting to incentivise them to do; and each employee may be different. So knowing this is key.
For example, is it just generally getting more income in, or is it more specific - like pushing retail sales, up scaling the service, or getting Clients to rebook within 6 weeks rather than 8.
What’s your most profitable service? Maybe incentivise this only.
Once you know, give your staff training on how to up sell, continually track how they’re doing and make the incentive clear and easy to calculate.
Hope that helps you a bit?