Considering from the outset how to help lessen tomorrow's legacy debt, maintenance and revision control.
Drupal is an open source project that consists of many thousands on modules. WIthin a single site, it is not unusual to see 100 modules working together. Each of these will receive feature and security updates over time as we as the updates to Drupal core itself.
It is essential to have a strategy to monitor and implement these updates that ensures the site functionality is not compromised by updating a module due to a security release.
As it is not possible to predict how many of these releases are likely in any given period, a robust maintenance and testing plan needs to be developed.