Use Handicap Stones
Advice on playing high handicap games as Black 
Handicap Stones permit severe moves
Use handicap stones when answering a corner approach
This is a common opening in 9 stone handicap games or in even games.
White plays a knight's approach move against a single corner stone.
Either the low extension at a or the high extension at b are the most common answers to the approach at in even games.
at c is standard and shares the corner with Black.
But in a 9 stone handicap game answering with the diagonal attachment at is better.
is a severe move, it claims the corner preventing White to play at c and pushes White towards the marked stone.
White answers with .
is a joseki move when the marked stone is in place.
strenghtens the corner and links with the left side star stone.
White would like to extend to the marked stone or at it's vicinity, making an ideal extension from and .
But White can only extend to resulting in a overconcentrated shape.
This result is better for White. White has good shape and establishes a base. Black didn't use his handicap stones to pressure White.
 The Second Book of Go, by Richard Bozulich