A reward after a certain section of the app is completed can be very motivating for the developer to get to the next stage. This keeps things moving. The staggered payments ensure that the developer does not get pissed off that they are not getting payments for the work they have completed. Also by having milestones and timelines to meet them, it means you can set your expectations before the development even starts and you do not have to be on edge all the time, learn more
https://www.helpware.com/dedicated-technical-support-outsourcing . Even if you do not set milestones at least establish a connect calendar so the developer gets in touch with you on specific dates throughout the project and give you a full update on the status of the project. Apart from keeping the developer motivated, pay as you go system allows for project approval points that will give you some peace of mind.