‘Uqbah ibn ‘Amir reported: The Messenger of Allah, ﷺ, said, “No Muslim performs ablution well and then prays two cycles with his heart and direction focused, except that Paradise will be necessary for him.”
الحضور القلب Mindfulness
"Al-hudur" or presence / peace of heart / mindfulness (must be there from the start of the prayer, physically and psychologically)
التفهم Comprehension
"At-tafahhum" or understanding / perception (you must understand what you are saying)
التعظيم Veneration
"At-ta3theem" - teaching your heart and mind that you are in front of Allah, "maintaining peace of mind, perception, and regulations in prayer".
الهيبة Reverence
"Al-hayba" - awe of Allah (love and fear mixed with recognition of who He is). This is a feeling which emerges from veneration and a deep respect for the All Powerful and Merciful Lord.
الحياء Shyness
"Al-hayaa" - shyness, being embarrassed or modest, feeling shame or sorrow for one's sins.
الرجاء Hope
"Ar-rajaa" - hope that Allah will accept our prayer and forgive us.
Imam Al-Ghazali says: “How many people respect a king and fear his authority but do not hope for anything from him? But it is very necessary for a servant to be hopeful of the reward of Allah ﷻ, the Glorious, [in response to his praise and supplications in ṣalât] as much as he is fearful of His punishment for [the servant’s] faults.”
“It is the believers who are successful: those who are humble in their prayer”
“Seek Allah’s help with patient perseverance and prayer. It is indeed hard except for those who are humbly submissive.”
Resources
Productive Muslim — Khushu: Humility in Salah Part 1 & Part 2
Practical Tips for Application — Satisfaction in Ṣalât Advice from Imam Abu Hamid Al-Ghazali