Attaining a CMMI level (maturity level / capability level) is not a one time job. Without a clear focus it could not be sustained. There are cases where CMMI HM compliant organizations degraded to ML 3 or ML 2 at  the time of next assessment due to improper maintenance of practices. Let me pinpoint certain points to ensure a smooth sustenance of CMMI HM

• Definitely first and foremost is senior management support. Then,
• There has to be  teams  responsible for  metric and improvement initiatives related activities.
• The team should be responsible for making periodical baselines, analysis of trends across baselines, fine tuning PPMs , target analysis  (baselines Vs targets), root cause analysis including analysis of common causes of variation, improvements identification and validation to meet the revised targets set by senior management.
• This team shall also be responsible for ensuring the implementation of process improvements within projects and analyzing the data from process improvements on a continuous mode, not merely at the end (at the time of target analysis)
•  Strong auditing practices needs to be ensured both at the project level as well as at the organizational lelvel. The progress of the above said team also should be audited.