Generalizing all lefties into one category is difficult, but I’ll do my best to explain some general strategies that would apply to most lefties.  I’m assuming that you (the reader) are right-handed.
 
Serve Return
When the lefty is serving a forehand serve from the backhand side, adjust your position slightly more to the right.  The lefty will often use a sidespin serve to curve the ball away from your wide forehand.  By standing more to the right, you will be able to better cover the wide forehand.
 
Down-the-Line Block

Your mental attitude has a TREMENDOUS influence over the outcome of the game. It can help you beat players rated much higher than you. However it can be a humongous factor in losses against players rated much lower than you. This can set back your goals in table tennis. So how do you fix it?

This year, I had one of my most devastating losses…
 
     I was playing at the 2013 US National Team Trials against 2x Olympian Khoa Nguyen.  I won the first 11-6.  I won the second 11-5.  I was winning 10-4 in the third game (best 4 out of 7 match) and lost that game and the match.  What went wrong…

 
(written by Matthew Henry)
“My times are in Your hand!” Psalm 31:15
Firmly believing that my times are in God’s hand, I here submit myself and all my affairs for the ensuing year, to the wise and gracious disposal of God’s divine providence. Whether God appoints for me . . . .
health or sickness,
peace or trouble,
comforts or crosses,
life or death–
may His holy will be done!