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Reviews and walkthroughs of intermediate software development products (models) are essential

Reviews and walkthroughs of intermediate software development products is necessary to ensure that errors which have been made are detected as early as possible in the software lifecycle.  Detecting and correcting an error later in the software development process becomes increasingly more expensive to manage.  Reviews and walkthroughs are good techniques to detect errors prior to full system implementation and coding. 

Testing alone will not guarantee product quality

Analysis shows that about 50% of software errors are likely to be detected by software testing alone.  This implies that other strategies must be used to reduce software defects, including explicit defect prevention (as opposed to detection and correction).  

Ensure models are not overly complex

Software can be made overly complex.  Use of measures such as McCabe's cyclomatic complexity can be used to identify complexity and to encourage reworking of complex code prior to its release.  It is better to produce maintainable (simpler) code at the development stage than for a maintainer to have to understand and amend source years after it has been written.

Consider handover procedures and use of the new system

Starting to use a new system requires careful planning, together with fallback and contingency procedures to handle unforeseen problems.

Software maintenance is ongoing

Software maintenance represents the bulk of the lifetime costs of a piece of software and, rather than be regarded as the exception, in reality it is the norm.  Indeed, the initial creation of a piece of software might be considered to be the exception.

Software maintenance occurs, some times because of poor coding and implementation; but by far the bulk arises from changing business requirements.