Something I've found that comes up often in my games is when I have groups sandwiching a narrow group:

Click Here To Show Diagram Code

[go]$$
$$ | . . . , . X . O . X . . . . . , . . . |
$$ | . . . . . X . O . X . . . . . . . . . |
$$ | . . . . . . . b a . . . . . . . . . . |
$$ | . . . . . . . c . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Assume that all stones are alive. What is the best endgame play for black? I can't kick at "a" because then when white descends to "b" he threatens a monkey jump. Playing underneath at "c" doesn't work either, since White has an excellent reply at "a". I never am quite sure what to do here or in similar situations.

The simple answer is, you can't settle this nicely in sente, which is what you seem to be trying to do.

Currently, it's very big for white to descend to b and make miai of th emonkey jumps (depending on exactly what else is on the board, of course). White can also just play the diagonal move and first line hane in both directions, which is probably bigger and more sente. If you want to stop that, you need to make a gote play. a seems okay, as below:

Click Here To Show Diagram Code

[go]$$B
$$ | . . . , . X . O . X . . . . . , . . . |
$$ | . . . . . X . O . X . . . . . . . . . |
$$ | . . . . . . 3 2 1 . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Click Here To Show Diagram Code

[go]$$B
$$ | . . . , . X . O . X . . . . . , . . . |
$$ | . . . . . X . O . X . . . . . . . . . |
$$ | . . . . 7 3 2 . 1 . . . . . . . . . . |
$$ | . . . . 5 4 6 . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

As the second diagram shows, white can of course play one of the big endgame sente points (well, almost definitely). But you can at least choose which side he gets it on, and so taking gote here might be a good enough endgame move to play.