Adhkaar is the plural of Dhikr; which can be translated as the remembrance of Allah. It involves simple yet highly rewarding sayings taught to us by the Prophet Muhammed (SAW). They draw us closer to our Rabb, distance us from Hell and wipes away our sins. It compensates for deficiency in one's salah. Adhkaar is encouraged at any time of the day but is more rewarding in the morning and evening. There are quite a few Adhkaar to be said in the morning and evening and I have included a few below and attached a link to an ebook for more.

• Ayatul Kursi (Q2:255)

"Whoever says it in the morning would be protected from the jinn until evening and whoever says it in the evening will be protected from them until the morning" - Saheeh At-Targheeb

• Al-Muawwidihat (Verses of Refuge) - these are the last three chapters of the Quran (Q112, 113 and 114) and whoever recites them three times in the morning and evening, they will be sufficient (as preotection) for him in everything (Abu Dawood)
• "I seek refuge in the perfect Words of Allah, from every evil created" - 3x in the evening 
• "O Allah, grant my body health. O Allah, grant my hearing health. O Allah, grant my sight health. No one has the right to be worshipped, except you." - 3x 
• "I am pleased with Allah as a Lord, and Islam as a religion and Muhammad (SAW) as a Prophet." - 3x and Allah will suffice him on the Day of Judgement
• "I ask Allah for forgiveness and to Him I repent." - 100x throughout the day
• "There is no diety worthy of worship except Allah alone, having no partners. To Him belongs all sovereignty and praise, and He is over all things Omnipotent." - 100x a day and the reward is likened to freeing 10 slaves, 100 good deeds written for him with 100 sins being removed and protection from shaytan. 

This is a great book and would help in completing and reviving this sunnah. It contains numerous invocations for different situations. You can find it under the reference section of the MuslimPro app for iOS and Android. I have also attached a link to an e-book similar to The Fortress of the Muslim here - The Key to a Successful Day