I assume you mean “sustainability of project results.”  In other words, the resulting benefits are not to be enjoyed just one time – they are recurrent in nature.  The best way to ensure that recurrent benefits are continually realized is to monitor the situation.  This would normally involve a proactive accounting system and someone to graph the on-going numbers.  Of course, it would also be assumed that management regularly reviews the data and “trigger points” are used to initiate corrective action.  Only in this manner can you ensure that recurrent benefits are continually “put in the bank,” so to speak.