Adi Shankaracharya used to perform Manas Pooja of God Shiva daily. In this stotra he has describes how he was performing Manas Pooja. Manas mean Mind. Pooja performed in the mind is called as Manas Pooja. 

Normally we perform pooja in the temple or by sitting in front of God’s idol. But at the time of manas pooja devotee of God will sit quietly in dhyanam position and with concentration he sees his beloved God in front of his third eye.