South London and Mausdley NHS Foundation- SLaM - providers of Mental Health Services 
The public can become Members of SLaM for free which means they will receive regular updates, be able to vote for Governors, take part in consultations.
The Involvement Register is a way for both service users and carers to become involved with in the planning and development of SLaM services. Those involved may help deliver training, sit on interview panels or committees. 
NHS Croydon's Care Commissioning Group - CCG - Made up of all 58 GP practices in Croydon this group is responsible for commissioning a range of health services on our behalf.  These will include services  at hospitals, in the community and mental health services. They hold Patient and Public Forums which are free and open to all. They also carry our survey's and hold consultations.
All GP practices are required to establish and support Patient Participation Groups. In many cases in Croydon practices these are well established groups that support the GPs to communicate with the local community and help to shape services. Other practices are at an earlier stage in the development of these groups. If you are interested in getting involved you should contact your GP practice directly but you can find out more information here.
Information about GPs PPGs
Healthwatch Croydon's role is to listen to the views of people in Croydon about health care services and share these with decision makers. They do this in a variety of ways; from formal consultations to reviews of NHS comment websites. Volunteers are key to their work, including their Enter and View work were trained volunteers are able visit and report on health and social care settings.
Healthwatch Croydon have recently released a report on Croydon's Mental Health Support Services and concluded that "Croydon’s mentally ill need stronger support services". Read the report.
Community and Voluntary Sector Representatives on Partnership Boards
CVA held elections in January 2016 to elect representatives to sit on a variety of boards and report back to the Croydon Voluntary Sector Alliance and relevant networks. Lee Buss and Laura Joanknecht are the elected representatives on the Mental Health Partnership Board.  It's the role of the representative to attend MH Partnership meetings and report back to the Mental Health Forum and to Croydon's Voluntary Sector Alliance.

You can also become a member of Croydon University Hospital to receive updates.
Patient Liaison and Advice Service - PALS
The PALS service is there to help sort out problems quickly as well as to help with formal complaints.
More general feedback can be given via the NHS Choices website