Shaykh Ṣālih Al-Fawzān (hafidahullāh):

Allāh commanded with steadfastness (upon the religion), moderation, taking the middle course and He prohibited harshness and negligence.

How is Istiqāmah Attained?

Steadfastness is (attained by ones actions and belief) being in conformity with the Book of Allah, the Sunnah of His Messenger (alaihi salātu was salām), and that which the Pious Predecessors from the early period of this nation were upon.

Source: Official Website of Shaykh Fawzān