Manners of Using the Mirror:

According to the reliable tradition related to Imam Ja'far-e-Sadiq (a.s.), the Holy Prophet (s.a.w.a.) had said that heaven is definitely rewarded to that young fellow who looks in the mirror and thanks God for the beauty given to him.
According to another tradition, the Holy Prophet (s.a.w.a.) kept on looking in the mirror while combing his hair and beard. He also ordered his wives and other companions to beautify themselves as God liked that a muslim should, before meeting another person, make himself acceptable to the other's eyes.
According to a tradition related to Hazrat Ali (a.s.), one should recite the following prayer while looking into the mirror.
‘All praise is worthy of that Allah who has created me and made my creation the best. He has made my face as it was the best possible to make. He has beautified me with such things, had these been found in others, they would have been considered as faults and then He has blessed me with Islam.’
_____________________________
From Allama Majlisi's Tehzeeb-ul-Islam Ch. 10 p 24