For the face-down cards, I started to open the cards two at a time, labeled 1 and 2. If the two match, I open the 3rd and the 4th and continue. If 1 and 2 don't match (which is very likely), I proceed to open the 3rd one. I then recall whether the 3rd matches with any of the previous ones (1 and 2) in any simplified form. If not, I open the forth and continue. This was the general strategy I followed.
Also, I found out that till the 9th card I will definitely have a match. This is due to the pigeon-hole principle. So, in the worst-case scenario every card from 1 to 8 has a corresponding match from 9 to 16 in some order. Thus when I open the 9th card it will definitely have a match for one of the 1-8 cards. This makes the strategy easier because I only have to memorize cards till 8 if there is no match. Otherwise, the game will get a lot easier if I luckily get a match before that