What are some Du'a which strengthen Imaan?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Adhering to Allaah's Orders, avoiding His Prohibitions in secret and in public and seeking to get closer to Him by performing the obligation He enjoined upon us are the most effective means to strengthen one's faith.  The Prophet said: "Allaah says (what means): “My Slave does not get closer to Me with anything more beloved to Me than something that I have enjoined upon him, and My Slave keeps getting closer to Me by performing Nawaafil (supererogatory acts of worship) till I love him, and if I love him, I become his hearing with which he hears, and his seeing with which he sees, and his hand with which he grips, and his leg with which he walks; and if he asks something of Me, I would surely give it to him, and if he takes refuge in Me, I would surely grant him refuge.” [Al-Bukhari]

One's faith increases and gets stronger to the extent of his obedience to Allaah's Commands and it decreases according to his disobedience.

Know that being in good company, befriending pious people and attending Islamic study circles also strengthen one's faith.

The Prophet used to supplicate Allaah frequently with the following supplication: "O Controller of hearts, make my heart steadfast on Your Religion." [Ahmad]

Allaah Knows best.